Side-chain alkylation of toluene with methanol to form styrene and ethylbenzene was studied over cesium ion-exchanged zeolite X (Cs-X) with various catalyst additives to find out effective catalyst systems for styrene formation as well as to elucidate the roles of the additives. Out of 13 types of additives examined, only ZnO exerted substantial positive effects on both toluene conversion and styrene selectivity. The role of ZnO was estimated to promote conversion of methanol to formaldehyde on the basis of IR study of adsorbed methanol. The formation of formaldehyde accelerated the formation of styrene as well as suppressed the transfer hydrogenation of styrene with methanol to form ethylbenzene. The positive effect of ZnO was also observed for Cs-X containing Cs+ in excess of ion-exchange capacity (Cs2O-Cs-X). The negative effect of additives such as Al2O3 was suggested to be caused by the formation of CO2 by water gas shift reaction and Bourdouard reaction. (C) 2016 Elsevier B.V. All rights reserved.