The investigation of the surface acidity in commercially available alumina-silicas through FTIR spectroscopy of adsorbed CO probe: molecules has been reported. The modification of gamma-alumina with silica led to the creation of both highly acidic Lewis and Bronsted acid sites (BASs); the former through isomorphous substitution of Si4+ ions by Al3+ ions at tetrahedral lattice sites; and the latter through formation of bridged hydroxy groups, similar to those found in zeolites. The relative strength and quantity of these sites reached a maximum with 40 wt.% silica content, above which the surface of the samples became silica coated and exhibited acidity approximating that of pure silica. This paper highlights (i) the suitability of CO as a probe molecule for the investigation of surface acidity, capable of differentiating between sites of very similar acid strength and/or coordination, and (ii) the enhancement of surface acidity in alumina through silica addition and subsequent formation of a mixed alumina-silica phase.