Catalytic supercritical water oxidation (CSCWO) is a technology that has been developed to treat aqueous waste streams. The addition of Pt/gamma-Al2O3 catalyst increased the reaction rates and complete conversion of pyridine was obtained at temperatures as low as 370 degrees C. The main products from this reaction were carbon dioxide, nitrous oxide, and nitrate ion. The presence of mass transfer limitations was investigated. Experimental results indicate the absence of external mass transfer limitations, whereas the reaction was limited by the diffusion of the reactants in the pores of the catalyst. The intrinsic rate of reaction, assuming power-law kinetics, was obtained. The diffusion coefficient of pyridine in a mixture of supercritical water and air was estimated, to allow the estimation of effectiveness factor and the Thiele modulus. The effectiveness factor decreased from 0.96 to 0.2 as the particle size was increased from 0.09 to 0.7 mm.