Like other standard-setters, he Marine Stewardship Council (MSC)’s ‘theory of change’ presumes that markets use this information to select for sustainable products, providing an incentive for producers to improve their practices and become certified. However, the underlying mechanisms which actually work to link market behavior and participation in the program in different contexts have not been systematically identified. The authors of this paper draws on broad MSC field experience to identify processes that have supported the theory of change in individual fisheries and develops a broadly applicable rapid assessment protocol, relying on a semi-structured interviews of key informants, to gather systematic evidence for key dynamics within the theory of change. The pilot test of the protocol demonstrates how the rapid assessment interviews allow program participants to relate their experience in their own words yet facilitate systematic comparison to identify common mechanisms within the theory of change. The authors propose its wider application to systematically advance our understanding of social and economic processes that drive of eco-label interventions in different geographies and supply chains around the world.