Niobia-phosphate aerogels, containing 5 and 10 mol-% phosphate, were prepared by a one-step sol-gel synthesis followed by supercritical drying with carbon dioxide. After calcination at 773 and 1073 K for two hours, these samples were characterized by nitrogen adsorption, X-ray diffraction, infrared study of adsorbed pyridine and 1-butene isomerization. The effect of calcination temperature on their acidic and catalytic properties, along with similar data for a sample prepared by immersing a niobia aerogel in phosphoric acid, allowed us to identify a surface phosphate with near P=O double bonds to be the acidity-enhancing species.