Chemical modification of a silica surface with chromium acetylacetonate (Cr(acac)(3), acac = C5H7O2) was carried out from the gas phase at 190 degrees C. This results in the formation of chromium acac species which interact with the silica surface because of hydrogen bonding between the surface hydroxyl groups and the quasi pi-electron system of two acac ligands of Cr(acac)(3). Ligand substitution in the chromium acac species on the silica surface pretreated at different temperatures was studied by IR spectroscopy, thermogravimetry, temperature-programmed oxidation, and chemical analysis. Water molecules and surface hydroxyl groups can replace acac ligands in the chromium acac surface species at 230 and 270 degrees C, respectively. The oxidation of the acac ligands in air occurs at 340 degrees C and is accompanied by conversion of Cr3+ to Cr6+.