Potassium phosphate supported on ion-exchanged CsX zeolite was prepared by impregnation method, and their performance was investigated for the side-chain alkylation of toluene with methanol. When K3PO4 was more than 5 wt%, the catalyst activity and selectivity of styrene and ethylbenzene were improved significantly. The conversion of methanol and selectivity of styrene and ethylbenzene reached 84.1 and 92.8% with K3PO4 loading of 7 wt%. It was concluded that moderate addition of K3PO4 decreased the amount of weak acid and increased the strength and amount of middle base sites. The base sites at 270-300 A degrees C (CO2-TPD) were favorable for this reaction. [GRAPHICS]