Abstract: The catalytic activity and the catalytic selectivity for p-xylenc in the o-xylene isomerization reaction at different temperatures over HZSM-11 with different SiO₂/Al₂O₃ ratios were measured by means of the pulse technique. The results show that the catalytic activity changes a little with the increase of the SiO₂/Al₂O₃ ratio.TPDchromatograms of ammonia adsorbed on HZSM-11 were obtained. The results show that the weak acid sites of HZSM-11 decrease with increasing the SiO₂/Al₂O₃ ratio of HZSM-11 but that the strong acid sites do not change obviously, Jn addition, we also obtained the distribution of acidic strength, and the lowest acidic strength at which the o-xylene isomerization reaction takes placc.