Source of selectivity of p-xylene formation in the disproportionation of toluene was studied on more than 20 kinds of the synthesized ZSM-5 using multiple characterization techniques. Based upon the total characterization and its comparison with the selectivity, we found that the selectivity in disproportionation could be correlated with not only the diffusion rate but also the external solid acidity, except for unusual species of zeolites. Unusual low or high selectivity was caused by the high external surface acidity or by the slow diffusion rate. It was considered that these unusual properties were based on the nonhomogeneous distribution of Al or Si in the zeolite. Source of the selectivity therefore, strongly depends on the physical and chemical zeolite property which is characteristic of each species of HZSM-5. An important experimental confirmation of the selectivity of p-xylene formation has been thus obtained.