Large pore zeolites, H-beta, H-mordenite (H-MOR) and H-omega, were dealuminated by steam treatment followed by acid leaching and were applied for transalkylation of toluene and 1,2,4-trimethylbenzene. The acidic properties of catalysts were examined by using TPD of ammonia and in situ FT-IR spectroscopy in the OH stretching region as well as pyridine adsorbed catalysts. XRD, mid-infrared spectroscopy and Si-29 and Al-27 MAS-NMR techniques were employed to investigate the structural changes of zeolites during dealumination process. Among the parent zeolites, H-beta and H-MOR showed high activity for transalkylation. H-omega showed very low activity although its acidity turned out to be high compared to that of H-MOR or H-beta. Over dealuminated samples, the activity was in the order H-MOR>H-omega approximate to H-beta unlike over the parent zeolites. It was observed that a large number of Bronsted acid sites were present in small pores and cages in H-MOR and H-omega. After dealumination, the activity and stability were substantially improved for H-MOR and H-omega but decreased for I-I-beta. H-MOR and H-omega tended to have more open structure and some of occluded acid sites became exposed to the main channels of 12-MR by dealumination. Such structural changes are believed to compensate for the loss of total acid sites to bring about the enhanced activity and stability over H-MOR and H-omega. Dealuminated H-omega, however, was deactivated rather rapidly. Consequently, H-MOR could be considered as a good acid catalyst for transalkylation of toluene and 1,2,4-trimethylbenzene after treatment by steam followed by acid leaching.