As a pioneer program using notion of PES, Costa Rican Payment for Environmental Services Program (PESP) has been analyzed as a very promising innovating instrument for conservation and has been considered as a reference for PES schemes. In fact, an important array of information and knowledge has been produced on the PESP by a variety of actors. Since his beginning, the PESP has changed (target groups, modalities...). This communication aims at providing a better understanding of the links between knowledge generation, learning processes and policy change. We identify six pathways of feedbacks, with internal or external dimensions. We show that despite the large external knowledge generation on PESP, the internal pathway linked to the classical Costa Rican policy process and control authorities is the most influential in PESP transformations. The communication invites to consider a multiple complex of learning process where both dimension, internal and external, are intertwined and interacting to explain policy changes.