Energy minimization methodology is used to study the interaction of substituted aniline molecules in various zeolites. The adsorption sites inside the zeolite channels and the diffusion characteristics of acylated products of 4-aminophenol are analyzed in detail. The selective formation of 4-hydroxyacetanilide, which is a pharmaceutically important compound, over various zeolites is studied. Three large-pore zeolites having 12-MR channel systems are selected : (i) zeolite-L with barrel-shaped cages, (ii) mazzite with circular pores and (iii) mordenite with elliptical pores and side pockets. The diffusion characteristics of the molecules are sensitive to pore architecture. The calculated diffusion energies do not indicate product selectivity in large-pore zeolites. Further, a study of diffusion inside the pores of ZSM-5, a medium-pore zeolite with IO-MR channel system, reveals that the C-acylated products have significantly larger diffusion energy barriers than the N-acylated products. The results are also useful in understanding the mode of interaction of the molecules with the zeolite framework.