The formation of 4,4'-dimethylbiphenyl is studied from the conversion of p-chlorotoluene over three types of reactions. The first type of reaction is the coupling of two p-chlorotoluene molecules by producing chlorine. The reaction is thermodynamically unfavourable and 4,4'-dimethylbiphenyl is not detected. The second type of reaction is comparable to a classical Ullmann reaction. It is shown that Cu supported on gamma-alumina is more reactive for the coupling of p-chlorotoluene than copper-powder. The third type of reaction is a catalytic alternative of the Ullmann reaction with hydrogen as reducing agent. The formation of 3,4'-dimethylbiphenyl is feasible. The activity and selectivity:are dependent on the type of metal and carrier used and also on the hydrogen partial pressure. The best catalyst is a Pd:supported on carbon. Bimetallic mixtures of Pd-Hg and Pd-Bi show also good performance. To our knowledge, it is the first time; that the formation of 4,4'-dimethylbiphenyl over typical hydrogenation/dechlorination conditions is reported.