Reduction of methylene blue with hydrazine was catalyzed by nearly mono-dispersed palladium particles of which diameters (2r(0)) ranged from 30 nm to 230 nm. The spectroscopically evaluated pseudo-first order reaction rate constants were proportional to r(0)(3 alpha), where 3 alpha varied from 2 to 3 with a decrease in the radius. The catalysis at large r(0) proceeded heterogeneously, whereas that at small r(0) occurred homogeneously. The relation between alpha and r(0) allowed us to evaluate the thickness of the reaction layer to be 20 nm. The analysis of the diffusion-reaction model demonstrated the thickness did not vary largely with r(0). (c) 2006 Elsevier B.V. All rights reserved.