The understanding of the porous structure of catalyst pellets and a realistic assessment of effective diffusitives is a problem of paramount importance in catalysis. Effective diffusivities have been correlated in terms of simplified models of pore structure. The availability of high speed computers has opened up the possibility of improved and more realistic models by considering the transport process in individual pores ultimately assembled into an appropriate overall description. In this paper, a two dimensional computational model is presented as a representation of pore structure. The calculation of effective diffusivities and a comparison with experimental results of other researchers indicate the adequacy of the model.