The motion of cellodextrin through the tunnel of processive exo-cellobiohydrolases is a fundamental step in the catalytic process by which these enzymes degrade cellulose to cellobiose and other small polysaccharides.  We have studied the movement of cellobiose, cellononose, and celloheptose in the reaction tunnel of Cel7A from Hypocrea jecorina (Trichoderma reesii) using molecular modeling and simulation to determine the mechanism of movement and the free energy profile of the movement. Using path sampling methods, we generated reaction paths with associated potential of mean force and have identified the mechanistic contributions to the barriers to motion and explore possible mechanisms for the leaving of cellobiose product and repositioning of cellodextrin chain for subsequent reaction.