Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state H-1 MAS NMR and Si-29 CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800 degrees C. For the catalysts treated at temperatures lower than 300 degrees C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 degrees C. The geminal silanols almost disappeared on the catalysts at 120 degrees C, while that for the silica gel occurred at 300 degrees C. Further increasing the calcination temperatures from 300 to 800 degrees C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.