The Sabatier principle states that optimal catalysis occurs when interactions between catalyst and substrate are of intermediary strength. Although qualitative in nature, this concept has proven extremely useful within (nonbiochemical) heterogeneous catalysis. In the current work, we show that the principle may be applied to an interfacial enzyme reaction. Specifically, we studied the breakdown of cellulose by different cellulases (wild types and variants) and found that the results could be rationalized in so-called volcano plots that are emblematic of the principle. This implies that the rate of the complex enzymatic reaction can be described by a single parameter (binding strength), and we show how this may help elucidating e.g. rate-controlling steps and relationships of substrate load and enzymatic efficacy. On a more general level, we propose that the Sabatier principle may be widely applicable to interfacial enzyme processes and hence open an avenue to the application within biocatalysis of some of the principles and practices originally developed for heterogeneous catalysis.