A series of In-loaded ZSM5 catalysts with Si/Al ratios of 17, 27 and 50 was studied. The catalytic activity for the NO reduction followed the 27 > 17 = 50 order. The acid properties were investigated by FTIR of pyridine as probe molecule desorbed in vacuum at different temperatures and by Temperature-Programmed Ammonium Desorption (TPAD) of NH4-In-ZSM5. The nature of the In-species was determined by the Perturbed Angular Correlation (PAC) technique using In-111 as a probe. The contribution of indium to the acidity nature of the samples seems to be important taking into account that the number of Bronsted acid sites was reduced after the In exchange. In the same way new and strong electron-donor acceptor sites were generated. The PAC results indicate that there exists an important fraction of the indium present in the active sample coordinated as in the In2O3 case, while another one corresponding to a non-well defined near-In-neighborhood, is present too in a small fraction.