The size-selective hydrogenation of butadiene-acrylonitrile rubbers (NBR) in carbon tetrachloride was studied using palladium particles dispersed in pores of different sizes. Synthetic porous smectite materials (SM) were used as supports and their pore sizes were changed with the synthetic conditions from 30 to 130 Angstrom compared with the average size of NBR polymers of 42 Angstrom in the solvent. At atmospheric pressure and at 50 atm of hydrogen, the hydrogenation of carbon-carbon double (C = C) bonds of NBR polymers proceeded by the Pd/SM catalysts with mesopores larger than 60 Angstrom. When the mesopores of catalysts are smaller than 40 Angstrom, in contrast, the reaction very slightly proceeds at atmospheric pressure and very slowly proceeds at 50 atm of hydrogen, respectively. The reaction is controlled by the pore sizes of catalysts, which determine the diffusion of the polymer molecules onto the dispersed palladium particles within the pores.