Mass balance is a well-established chain-of-custody approach in the chemical industry and beyond. It enables certified and non-certified materials to be physically mixed throughout the value chain but kept separate through verifiable bookkeeping. As outlined in our “Mass Balance Explained” article, mass balance is essential for integrating alternative certified feedstock into existing industrial infrastructure and thus scaling circularity.
In the context of chemical recycling, an additional layer becomes relevant: attribution. While mass balance and attribution – especially the concept of free attribution – may appear interchangeable at first glance, they are not. The distinction is key to understanding why different attribution approaches exist under ISCC PLUS and why they are necessary.
Mass Balance: Controlling Quantities
Mass balance addresses general quantitative and process-related questions: how much certified input material enters a system, and how is it tracked through the production process?
Mass balance ensures that certified inputs are physically received by the supply chain, that losses are accounted for (e.g. via conversion factors), and that the total amount of certified input is consistently tracked within the system. In this way, mass balance provides the accounting framework that links inputs and outputs and prevents double-counting. However, it requires clear guardrails for how the sustainability characteristics of certified inputs can be attributed to specific outputs in complex production systems. This is where attribution comes into play.
Did You Know?
Under ISCC PLUS, mass balance can be implemented using the Rolling Average Percentage method and the credit method. They differ in how certified input is reflected at the output level.
The Rolling Average Percentage method expresses certified content as an average share across outputs over a defined period. The credit method, by contrast, allows certified inputs to be translated into discrete credits that can be attributed to specific outputs. The attribution approaches discussed in this series apply to the credit method, which is particularly relevant for chemical recycling.
Attribution: Making Choices
Attribution addresses the detailed question: to which outputs can the sustainability characteristics of certified inputs actually be assigned?
A Brief Historical Digression
Historically, attribution received little attention because mass balance was often applied in straightforward production systems or in voluntary markets without differentiation between end uses. In sectors such as food production, processes typically yield a single main output, and certified inputs are not routed into competing production pathways.
Back to Today
Chemical recycling changes this dynamic: production processes often generate multiple outputs that enter very different downstream value chains. Some chemically recycled outputs are used as materials, others as fuels, and some can be used for both depending on the supply chain. As EU and global regulatory frameworks increasingly distinguish between material recycling and energy recovery, attribution becomes essential. Choices now affect how recycled content is attributed, claims are made, and alignment with recycling definitions is ensured. This must be done in a clear and transparent manner, with harmonised attribution rules set and verified by certification systems to ensure a level playing field as chemical recycling gains in importance.
One Mass Balance System, Multiple Attribution Approaches
Three attribution approaches can be applied under ISCC PLUS. These are not competing concepts, but rather responses to different market realities that reflect a variety of balances between flexibility, regulatory alignment and robustness.
- Free Attribution allows flexible attribution of certified inputs to outputs within defined guardrails and serves as a starting point for voluntary markets.
- Fuel-Use Excluded Attribution restricts the attribution of certified inputs to material uses and aligns chemical recycling with regulatory definitions of recycling.
- Proportional Attribution assigns certified inputs to all outputs strictly in proportion to their share of the certified input, thereby eliminating choices.
Looking Ahead: Attribution Approaches Explained in Detail
Making attribution rules explicit is not about adding complexity for its own sake. It is about ensuring that certification systems remain transparent, credible and aligned with evolving market and regulatory expectations.
This distinction between mass balance and attribution provides the foundation for the next articles in this series. In the following pieces, we will examine the different attribution approaches under ISCC PLUS in more detail, starting with free attribution as a practical entry point for chemical recycling in voluntary markets.
Our Chemical Recycling Series
You have just read the second article in our chemical recycling series. Start from the beginning:
