The transalkylation of m-diethylbenzene with benzene to ethylbenzene has been studied over some beta- and Y-zeolites at 210-270 degrees C and 48 bar total pressure. A rather complex mechanism has been found for the reaction, which is accompanied by the formation of several undesired byproducts, especially over Y-zeolites. No direct formation of ethylbenzene from m-diethylbenzene was observed. Only o- and p-isomers can be transformed into the desired product. Furthermore, the latter isomers do not isomerise directly to the other form. Naphthalene forms very likely from o-diethylbenzene only, while diphenylethanes and methylethylbenzenes seem to form preferentially from m-diethylbenzene. A simplified general reaction scheme is proposed and the kinetic parameters for such reactions were estimated, together with their Arrhenius parameters.