The reaction of ZrCl4 vapor at 300, 450, and 600 degrees C with silica and gamma-alumina preheated at 300 and 600 degrees C was studied by XRD, FTIR, H-1 MAS NMR, and chemical etching with sulfuric acid. NMR and FTIR revealed a consumption of OH groups in the reaction. However, subsequent water vapor treatment brought some of the OH groups of the support back, indicating that part of the surface had been directly chlorinated. The ratio between permanently consumed OH groups and directly chlorinated OH groups depended on the reaction temperature. OH groups were permanently consumed in mono- and bimolecular reactions of ZrCl4, leading to the formation of isolated M-O-ZrCl3 and (M-O)(2)-ZrCl2 species (M = Al or Si), respectively. Direct chlorination was associated with the formation of crystalline ZrO2 agglomerates, as detected by XRD in the samples prepared at 450 and 600 degrees C. These agglomerates were insoluble in sulfuric acid. Although at the reaction temperature of 300 degrees C the samples were amorphous in XRD, the reappearance of some OH groups on the original surface after water treatment, together with etching tests, suggested that agglomerates were also formed at that temperature. In addition to agglomeration and exchange reactions with OH groups, on 600 degrees C alumina ZrCl4 dissociated to (Al-O) pairs. This led to a higher Zr saturation density and Cl/Zr ratio on alumina than on silica. The reactions of the HCl evolved in the main reactions were considered minor. Increasing the preheat and reaction temperatures decreased the Zr concentration on the surface and thus diminished the surface coverage. The surfaces of the ZrCl4-modified and water-treated silica and alumina surfaces consisted of Zr-OH groups formed in the hydrolysis of isolated ZrClx species, original OH groups of the supports, and ZrO2 agglomerates formed in the vicinity of siloxanes and (Al-O) pairs.