The dehydrogenation of diethylbenzene to divinylbenzene is a catalytic reaction. The catalyst for the dehydrogenation was prepared by co-precipitation of iron and chromium hydroxide from nitrate solution, followed by doping with potassium carbonate and drying. To make available the internal surface area of the catalyst for the reactant, the pores must be of the proper sizes to allow the reactant to diffuse and penetrate inside the catalyst pellets. The prepared catalyst was considered as a model for investigating the role of diffusion in catalyst design. In this study, different mechanisms of diffusion, such as Knudsen and bulk, were investigated for the case of diethylbenzene diffusion into the catalyst and it was concluded that the pore sizes should be in a range that permits transitional diffusion (both Knudsen and bulk diffusion). The catalyst grain size can be controlled and varied by acting on parameters such as the speed and time of mixing, type of alkali, temperature and pH. Particle size distribution experiments were conducted for different types of alkali and speeds of mixing in order to characterize the catalyst. The effects of the grain size, formed during co-precipitation, upon the pore size distribution of the catalyst pellet which affects the effective diffusivity were discussed. The pore size distribution of the model catalyst was obtained and the effective diffusivities were calculated by numerical integration of the Johanson-Stewart equation.