The oxidation of diethanolamine to N-(2-hydroxyethyl) glycine in alkaline solution was carried out over a Pd-Al₂O₃ catalyst in a liquid-full reactor, and the effect of mass transfer on the global rate of reaction was studied. Data were taken at 40°C and atmospheric pressure. Experiments were first done to determine the order of reaction with respect to oxygen, which was found to be 0.64 for an assumed tortuosity factor of 3.4 for the catalyst particles. Under the reaction conditions used, the effective mass transfer area between liquid and porous catalyst was found to be less than the outer surface area of the particles. With 0.5% Pd-Al₂O₃ catalyst, the global rate of reaction was mainly controlled by liquidsolid and intraparticle mass transfers, and the reaction was considered to be completed inside the deactivated outer shell, as the core of the particle still retained high catalyst activity.