In our previous article from this series, we introduced the distinction between mass balance and attribution. Mass balance monitors how much certified input material enters a system and tracks it through a production process. Attribution determines which outputs receive the sustainability characteristics of those certified inputs.
Under ISCC PLUS, several attribution approaches exist to address different industry needs, levels of market readiness, and regulatory expectations. The first of these approaches is free attribution, which, together with clear guardrails, provides a starting point for complex systems in voluntary markets.
Free Attribution in Practice
Under free attribution, sustainability characteristics originating from certified inputs can be flexibly attributed, within defined guardrails, to one or more outputs of a production process.
A common example is found in chemical recycling value chains. When plastic waste is converted into pyrolysis oil through chemical recycling, it can then serve as a certified alternative feedstock for large industrial plants such as steam crackers. These plants then mix and process fossil and alternative feedstocks and produce several chemical intermediates simultaneously.
Imagine a cracking process, where both fossil feedstock and ISCC-certified circular feedstock derived from chemically recycled plastic waste are introduced. The plant produces several outputs – for example:
- 100 tonnes of fuel output.
- 100 tonnes of product A, used in plastic manufacturing.
- 100 tonnes of product B, used in plastic manufacturing.
- 200 tonnes of a dual-use output that can be used either as chemical feedstock for products or as fuel, depending on the downstream application.
Prior to attribution, the losses must be accounted for via a conversion factor. In such a process, part of the feedstock may be used to generate process energy, converted into CO₂ through side reactions, or end up in residues. Suppose that 100 tonnes of certified content remain attributable after deducting these losses.
In reality, the certified atoms are distributed across all output streams. Under free attribution, however, the operator can flexibly decide which output stream(s) receive the certified share, within the mass balance attribution guardrails.
For example, the 100 tonnes of certified input could be attributed (via mass balance approach) entirely to product A, which can then be forwarded as ISCC-certified circular material. Alternatively, the same certified input could be attributed to product B, split between both products, or be attributed to the other outputs.
As the name suggests, attribution under this approach is flexible within defined guardrails. What remains constant is that the overall balance of the system must be maintained: the total amount of certified output cannot exceed the certified input entering the system (with the losses accounted for), and all attributions are verified through the mass balance system.
The Role of Chemical Connectivity
One key guardrail for mass balance attribution under ISCC PLUS is chemical connectivity, which requires that the certified input can plausibly contribute to the certified output within the processing unit. In practice, this means that the certified input must be chemically capable of contributing atoms to the certified product through a technically plausible production pathway.
Returning to the steam cracker example: intermediates such as product A, product B, the fuel output, and the dual-use output can receive certified attribution because they are direct products of the cracking process. By contrast, certified content shall not be attributed to a product where there is no link (no chemical link or no plausible infrastructure) between the certified input and the product.
This requirement ensures that sustainability characteristics remain linked to the process chemistry and cannot be assigned arbitrarily across unrelated production streams.
At a Glance: Guardrails Under Free Attribution
- Certified inputs must be physically received at the site.
- Attribution must occur within the defined system boundary (processing unit).
- Process losses must be reflected through conversion factors based on actual production data.
- The total amount of certified content attributed across all outputs cannot exceed the certified input.
- The amount of certified content attributed to an individual product cannot exceed its physical production volume over the mass balance period.
- Chemical connectivity between the certified input and output must be in place.
Would you like to dive deeper into free attribution under ISCC PLUS?
You can find more information in the ISCC PLUS Mass Balance Guidance Document (1.0), which has recently undergone public consultation. The final version is expected in June 2026.
Why Flexibility Matters in Early Market Development
In the broader circular economy, chemical recycling is complementary to mechanical recycling and forms part of a wider set of solutions for managing “difficult to recycle” plastic waste streams. However, chemical recycling supply chains are still evolving. In many cases, chemically recycled alternative feedstock represents only a small share of the total input processed in existing infrastructures. At this stage, the ability to integrate even small amounts of certified circular feedstock into existing infrastructure is an important first step.
The free attribution approach under ISCC PLUS reflects this reality. It provides a practical starting point that allows companies to begin incorporating alternative certified feedstocks into complex industrial systems. Such flexibility can help create early incentives for the use of certified circular inputs and supports the gradual scaling up of these materials within established value chains for the voluntary markets.
As markets mature and regulations develop, additional attribution approaches will introduce further constraints on how certified inputs can be attributed across outputs.
Looking Ahead
One such approach is fuel-use-excluded attribution, which introduces an additional distinction between the material and energy uses of process outputs. In the next article, we will examine how this attribution approach works and how it aligns more closely with regulatory definitions of chemical recycling under the mass balance approach.
Our Chemical Recycling Series
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