Acetophenone in aromatic solvents may undergo two different processes in the presence of large pore acid zeolites as catalysts such as Y, beta and Omega. Thus, acetophenone and toluene react with high conversion to give a complex mixture containing predominantly methyl derivatives and positional isomers of 1,1-diarylethanes, together with dypnone, 1,3-diphenylbutadienes and minor amounts of 1,1,1-triarylethanes, The formation of diarylethanes closely resembles the previously reported product distribution pattern for the analogous reaction using aromatic aldehydes. In contrast, in benzene or tetrachloromethane the aldolic condensation of acetophenone to yield dypnone was the maj or pathway, Although dypnone in toluene also reacts to form diarylethanes, we have established that the reaction with toluene and the aldolic condensation are independent processes, Influence of zeolite crystalline structure and catalyst particle size on the course of the reaction reveal that a severe diffusion restriction is controlling the conversion and the selectivity of this process. Accordingly, Omega was found a very inefficient catalyst, while at similar conversions beta was much more selective to the less-space demanding dypnone than the Y zeolite.