Condensation of acetaldehyde with ortho-xylene was carried out in the presence of solid acid catalysts, in spite of the strongly competitive autocondensation of acetaldehyde to give crotonaldehyde. A semibatch reactor, with a continuous feed of paraldehyde into a stirred system containing o-xylene and the catalyst, proved more convenient to obtain 1,1-bis(3,4-di-methylphenyl)ethane, because it assures a high o-xylene/acetaldehyde ratio. High selectivity was found in runs at 140 degrees C, over zeolitic catalysts of type Y (dealuminated) or beta. Some catalyst deactivation was observed and a partial regeneration was found possible.