As part of a systematic study of solid acids, a series of high surface area single-phase alumina-titania mixed oxides were prepared. These materials were found to have higher surface areas, greater porosity and enhanced specific surface acidity than comparable alumina-titania solid acids reported in the literature. The thermal stability of the phases and their textural and physicochemical properties have been studied over a range of Al/Ti compositions and for calcinations up to 1000 degrees C. The samples were characterized by differential thermal analysis-thermogravimetry, X-ray diffraction, nitrogen physisorption and X-ray photoelectron spectroscopy. Surface acidity was measured by temperature programmed desorption and in situ Fourier transform-infrared spectroscopy, using ammonia as a probe molecule. The strength and density of the acid sites were proportional to the Ti-content.