Alkylation of benzene, toluene, o-xylene, and biphenyl was carried out over aluminum halides and ion-exchange resin Amberlyst 36 Dry with the use of branched C-5 alkylating agents. Tertiary product selectivity is related to a catalytic system of aluminum halides with reduced activity or ion-exchange resin Amberlyst 36 Dry. The high selectivity of branched secondary products is provided by an equilibrium condition. The equilibrium in a liquid phase of positional and structural isomerization and transalkylation at 303-363 K were studied. The prediction method of an isomerization equilibrium was recommended for C-5 branched alkylbenzenes.