Catalytic conversion of large molecules, such as heavy petroleum residue and coal-derived liquids, involves the restricted pore-dif- fusion transport of such molecules into the catalyst pellets. The sizes of molecules in these feedstocks range between 25 X m and 150 X 10-lo m. The largest fraction is around 50 X m (Ruckenstein and Tsai, 1981). The degree of restriction of these molecules by the pore wall has been the subject of many investigations (Colton et al., 1975; Prasher and Ma, 1977; Prasher et al., 1978). The catalytic conversion of large molecules, optimal pore size of catalyst pellets, has been reported for heavy petroleum residue (Eigenson et al., 1977; Ohtsuka, 1977) and for coal-derived liquids (Brooks et al., 1976; and Yet et al., 1976). In this paper we shall investigate the optimal pore size for these feedstocks with the as- sumption of first-order chemical kinetics (Sapre et al., 1980 Schuit and Gates, 1973). The interaction between diffusing molecules and the surface may play an important role in catalytic conversion of large molecules. In fact, Glandt (1981) has shown that the con- centration is higher in the periphery of the pore than at the center. This interaction, however, is ignored in this work because the emphasis of this paper is on the interplay among diffusion, reaction and the nonlinear partition coefficient. Such interaction could be incorporated, but the mathematical analysis would then be very difficult.