Liquid-phase benzylation of o-xylene to 3,4-dimethyl diphenylmethane (3,4-DMDPM) with benzyl chloride (BC) has been investigated in the presence of various zeolite catalysts. Conventional homogenous Lewis acid catalyst, AlCl3, is also included for comparison. Under identical reaction conditions, zeolite H-beta is considerably more selective than the other zeolite catalysts and AlCl3 in the benzylation of o-xylene. The conversion of BC. rate of BC conversion and selectivity to 3,4-DMDPM over H-beta after Ih of reaction time and at 363 K are ca. 33.5 wt.%, 12.6 mmol g(-1) h(-1) and 92.2 wt.%, respectively. For comparison, the conversion of BC, rate of BC conversion and selectivity to 3,4-DMDPM over AlCl3, under identical reaction conditions, are estimated to be 76.6 wt.%, 28.8 mmol g(-1) h(-1) and 65.5 wt.%, respectively. Higher amounts of consecutive products (32.9 wt.%) are achieved over non-shape-selective AlCl3 as compared to the zeolite H-beta (6.9 wt.%). The conversion of BC to the products largely depends on the reaction conditions and acidity of the zeolite catalysts. Acidity of the zeolite catalysts is measured by temperature-programmed desorption (TPD) of ammonia. The effect of reaction time, SiO2/Al2O3 ratio of H-beta, catalyst concentration, reaction temperature and o-xylene to BC molar ratio on the catalyst performance is examined in order to optimize the conversion of BC and selectivity to 3,4-DMDPM. The conversion of BC using H-beta is increased significantly with the increase in reaction time, catalyst concentration, reaction temperature and o-xylene to BC molar ratios, however, an increase in the SiO2/Al2O3 molar ratio of H-beta lowered the BC conversion. The benzylation of m-xylene and p-xylene is also investigated over zeolite H-beta catalyst. H-beta is recycled four times and a decrease in BC conversion is observed after each cycle which is related to the minor dealumination of zeolite catalyst by HCl formed during the reaction. The formation of DMDPM is explained by an electrophilic attack of the benzyl cation (C6H5CH2+) on the o-xylene ring whose formation is facilitated by acid sites of the zeolite catalysts.