The ambient temperature adsorption of acetonitrile tin its da deuterated form, to avoid Fermi resonance band splitting in the analytical v(CN) spectral region was studied on pure gamma-alumina, gamma-alumina doped with two different amounts of sulfates, gamma-alumina doped with Ca, and a Ca-doped alumina that was thermally transformed into the delta,theta-alumina form. Acetonitrile adsorption readily reveals the different Lewis acidity due to different surface cationic species (e.g.,Al3+ and Ca2+) but is only partly sensitive to the acid strength heterogeneity exhibited by Lewis sites due to the same cationic species (Al3+ ions, in the present case). Acetonitrile uptake perturbs all surface hydroxyl species that are free from H-bonding of the OH OH type by either ligand insertion (the OH species at similar to 3780 cm(-1)) or by (weak) H-bonding (the OH species at similar to 3740 cm(-1)). When a plain H-bonding is formed, the relevant v(CN) mode is hardly distinguishable from that of a liquidlike physisorbed phase. The presence of large amounts of surface sulfates induces weak Br phi nsted acidity in some OH species, whose H-bonding interaction with acetonitrile yields a v(CN) band upward shifted by some 10 cm(-1) with respect to liquid nitrile. Acetonitrile adsorption reveals that the presence of sulfates increases the surface Lewis acidity of alumina, mainly by increasing the relative amount of the stronger family of Lewis acid sites. On undoped alumina, acetonitrile undergoes a slow hydrolysis reaction (leading to acetamide species) and a slow polymerization reaction (starting from a CD2CN- anion precursor). When abundant surface sulfates are present on alumina, nitrile reactions of both types are totally hindered, whereas the presence of Ca enhances the surface reactivity of acetonitrile,especially when the thermal transformation of gamma-alumina into delta,theta-alumina increases the surface concentration of Ca dopant species.