The surface hydroxyl content and surface structure of silica and other oxides with and without surface modification were systematically studied by solid state Si-29 NMR, thermogravimetric analysis, and the lithium alanate method. Aerosil 90 as a well described reference system and functionalized zirconia-silica particles were used in the validation of the lithium alanate method. 3-Methacryloxypropyltrimethoxysilane and dodecylphosphonic acid were applied as surface modifiers. The determination of silanol content of Aerosil 90 by Si-29 NMR and TGA confirms the results obtained by the lithium alanate method, which also allows for the determination of the remaining surface hydroxyl content after surface modification. For both silane coupling agents, the residual hydroxyl content of modified zirconia-silica is decreased by a factor of approximately 2 compared with that of the unmodified mixed oxide, whereas after modification with dodecylphosphonic acid, the hydroxyl content is slightly higher. These results are again in good agreement with those by Si-29 NMR confirming that the lithium alanate method is a reliable and easily practicable method for surface hydroxyl determination. (C) 2012 Elsevier Inc. All rights reserved.