Enzymatic deconstruction of cellulose occurs at the aqueous/cellulose interface. Most assays to explore cellulase activity, however, are performed in bulk solution and, hence, fail to elucidate surface-reaction kinetics. We use flow ellipsometry to quantify the adsorption and surface reactivity of aqueous cellulase on a model cellulose film substrate. The rate of cellulose digestion at the aqueous/solid interface increases with increasing bulk concentration of enzyme, but only up to a plateau corresponding to the maximum adsorption density of cellulase. Kinetic data are analyzed according to a modified Langmuir-Michaelis-Menten framework including both reversible adsorption of cellulase to the cellulose surface and complexation of surface cellulose chains with adsorbed cellulase. At ambient temperature, the molar turnover number is 0.57 +/- 0.08 s(-1), commensurate with literature values, and the Langmuir adsorption equilibrium constant, characterizing the binding strength of the cellulase, is 0.086 +/- 0.026 ppm(-1). The rate-determining step in the surface-reaction sequence is complexation of adsorbed cellulase with the solid-cellulose surface. Simultaneous knowledge of sorption and digestion kinetics is necessary to quantify cellulose deconstruction.