The basic properties of nitrided galloaluminophosphate "AlGaPON" are documented by FTIR of sulphur dioxide and deuterated chloroform. To evidence the increase in the number of basic sites with nitrogen enrichment, we compared the spectra recorded after adsorption of CDCl3 on the phosphate precursor "AlGaPO" and on four oxynitrides with increasing nitrogen contents. On the oxide, adsorption arises from CDCl3 interacting with surface hydroxyl groups, on the oxynitrides basic sites are shown to be M-NH2 groups. This interpretation is confirmed by FTIR of adsorbed SO2.