Potentiometric titration data from pure rutile, anatase, and a commercial fumed titania (Degussa P25) were analyzed in terms of proton binding isotherms from which proton affinity distributions (PADs) of surface sites were obtained. As-received samples, whose thermal and storage history were not systematically controlled, as well as samples subjected to controlled calcination-rehydration-drying treatments were studied. The results indicated the occurrence of a limited number of surface groups on the two polymorphs. The behavior of pure rutile and anatase could be admired to simulate the acid-base behavior of the commercial sample; on this basis the surface of fumed titania consists largely of anatase-like structures with a small contribution (7%) of rutile-like groups. The region of v(OD) stretching vibrations of isolated -OD groups on extensively dehydroxylated samples was found to correlate with the pK’s determined from PADs, A qualitative assignment of measured pK values based on either the original MUSIC model (Hiemstra, T., de Wit, J. C. M., and Van Riemsdijk, W. H., J. Colloid Interface Sci. 133, 105 (1989)) or a refined version of it is presented.