Enzymes adsorbed at aqueous/fluid and aqueous/solid interfaces play a governing role in many technological applications including two-phase biocatalysis, protein-stain removal, and cellulose deconstruction, among others. In addition to solution kinetic measurement of product formation, optical techniques such as ellipsometry and optical waveguide spectroscopy are applied to measure directly substrate consumption. By immobilizing solid substrates on optically smooth supports, such as silicon wafers, substrate thickness decline is directly monitored in response to enzyme concentration, solution environment (e.g., pH, ionic strength, and temperature), and surfactant additives. Three example cases are discussed: cleavage of mandelonitrile at the aqueous/organic liquid interface by hydroxynitrile lyase; cleavage of immobilized ovalbumin at the aqueous/protein interface by Subtilisin Carlsberg; and cleavage of crystalline cellulose at the aqueous/solid interface by endoglucanses, exoglucanases, and their mixtures. In all cases, adsorption behavior of the enzyme is paramount. Cleavage kinetics obeys a Langmuir-Michaelis-Menten framework in which the adsorbed enzyme concentration controls the reaction rate. Accordingly, cleavage rates plateau with increasing bulk enzyme concentration according to enzyme adsorption saturation. Longer ageing times and/or higher cross-linking densities of the substrate slow cleavage kinetics due to enzyme inaccessibility. Surfactants slow bulk aqueous proteolysis due to steric blockage arising from surfactant/protein complexation. Conversely, surfactants can increase interfacial cleavage rates, primarily by swelling of the immobilized substrate. It is not possible to extrapolate measurements of bulk aqueous catalysis rates to those occurring at an interface. In addition to improving active-site cleavage rates, effective enzymes for interfacial catalysis should be engineered to adsorb densely, but reversibly.