Thermogravimetric analysis (TGA) is investigated for determination of the OH surface density (OH/nm(2)) and carbon content of silica and titania powders made by flame aerosol and sol-gel processes. It is shown that it is possible to distinguish between physically adsorbed and chemically bound water and to rapidly determine the OH surface density even of small powder samples (< 0.2 g) by TGA calibrated with LiAlH4 titration data. The high accuracy of the OH surface density determination by TGA is confirmed further with additional LiAlH4 titration data of silica powders and by comparison with the specifications of commercially available silica Aerosil and titania P25 powders. Furthermore, by connecting a CO2 sensor or a mass spectrometer to the TGA balance, it is possible to verify the carbon content and determine other components (organic residues) of the powders. Thereby, it is shown that flame-made powders have high purity while the preparation conditions of sol-gel powders greatly affect their purity. At a water-to-precursor ratio of 1000:1, no organic residues are detected in sol-gel powders, while lower ratios result in residues in the product powder from the precursor, solvent, or catalyst.