The present work deals with the acid properties of silica, alumina and two mixed oxides obtained by grafting silica on alumina (SA sample) or alumina on silica (AS sample). The. surface acidity of the samples, of both Lewis and Bronsted type, was determined by means of microcalorimetry and temperature programmed desorption (TPD), using pyridine and 2,6-dimethylpyridine as probe molecules. Both techniques point out that the grafted mixed oxides SA and AS have acidic properties different from those of the pure alumina and silica starting supports. Some differences exist, in terms of acid sites strength distribution, between TPD and calorimetric results, which can be reasonably attributed to the different activating conditions of the samples.