Three MFI-type, H-GaZSM-5 gallosilicates were synthesized with Si/Ga ratios of 25, 50, and 75, respectively. Incorporation of gallium into the zeolite framework was checked by powder X-ray diffraction, which showed an approximately linear variation of d spacings with Si/Ga ratio. Fourier transform IR spectroscopy of CO and N-2, adsorbed at liquid nitrogen temperature, was used to characterize these gallosilicates after activation in vacuo at temperatures from 673 to 973 K. It was found that both probe molecules formed stable adducts with polarizing centers having either Bronsted or Lewis acid character. On samples activated at a temperature smaller than 700 K Bronsted sites were found to be predominant; apart from regular Si(OH)Ga sites, adsorbed carbon monoxide also revealed the presence of a small fraction of Bronsted acid groups associated with nonregular sites. These could be located either at the mouth of small cavities (presumably associated with defective regions of the zeolite framework where gallium was segregated) or on amorphous regions. On the sample with Si/Ga = 25 activated at temperatures higher than 700 K, gallium showed a marked tendency to go into either partial or total extraframework positions, thus generating coordinatively unsaturated Ga3+ centers with strong Lewis acid character. These centers were revealed by formation of CO and N-2 adducts where stretching frequencies of the adsorbed molecules showed a strong hypsochromic shift, Delta nu. Values of Delta nu as high as 82 and 25 cm(-1) were found for CO and N-2, respectively. In the case of dinitrogen, interaction with Lewis acid centers also bring’s about a pronounced increase of IR band intensity, and this effect can advantageously be used to monitor extraframework species.