In this work, a concise review on enzymatic hydrolysis of cellulose by endoglucanase is presented. In addition a novel population balance based kinetic model for the action of endoglucanase has been developed. One important feature in the proposed model is a new approach to consider the evolution of cellulose chain length. In addition, a new mechanism for hydrolysis of cellulose by endoglucanase is proposed in which bond breakage is a function of position in the chain as well as the susceptibility to enzymatic attack. In the first part, the focus is on developing a mechanistic model for the action of endoglucanase and then in part II a mechanistic model for the action of exoglucanase has been developed together with a holistic model considering all the major steps involved in enzymatic hydrolysis. In the case of endoglucanase action, the model prediction is very close to the experimental results presented by Kurakake et al. [12]. Using the estimated values for the parameters (confidence interval = 98%), the evolution of size distribution for cellulose polymers during the course of hydrolysis has been presented. (C) 2011 Elsevier Ltd. All rights reserved.