Solid-state nuclear magnetic resonance spectroscopy was used to investigate the coordinative states of surface Al species on various alkylaluminum-modified Phillips CrOx/SiO2 catalysts. The alkylaluminum-modified Phillips CrOx/SiO2 catalysts were examined via ethylene homopolymerization. 1H and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra clearly demonstrated that the existing states of surface Al species in alkylaluminum-modified catalysts strongly depended on the type of alkylaluminum cocatalyst, concentration of alkylaluminum and the calcination temperature. 1H MAS NMR spectra of alkylaluminum-modified Phillips CrOx/SiO2 catalysts, calcined at two different temperatures, exhibited similar trends in peak shift. 1H spectra showed that with an increase of Al/Cr ratio and calcination temperature, the main peak shifted to high field, indicating that the dominant surface proton species changed from hydroxyl to ethoxyl and ethyl groups. 27Al MAS NMR spectra showed the presence of three different coordination states (6-, 5-, and 4-coordinated Al species) in the alkylaluminum-modified Phillips catalysts. In comparison of different alkylaluminum cocatalysts, it was found that the reactivity of alkylaluminum modified Phillips catalyst decreased in the order of TEA>DEAH>DEAE. The amount of 4-coordinated Al species of Phillips catalysts modified by TEA, DEAE and DEAH also decreased in the order of TEA>DEAH>DEAE, indicating that the presence of 4-coordinated Al species is related to the polymerization activity.