The Phillips family of one-component catalysts, created by the interaction of a inorganic chromium compound such as CrO3 and an oxide support, typically silica, initiates ethylene polymerization in the absence of alkylaluminum. activators. Despite intense scrutiny since its discovery over four decades ago, the nature of the active site has remained obscure and the mechanism of olefin polymerization ill-defined, Spectroscopic studies of the working catalyst are hampered by low active site densities. To circumvent this problem, we have prepared well-defined organochromium fragments, supported on silica, which manifest remarkably similar activity to the CrO3/SiO2 catalyst, and produce similar high-density polyethylene. The uniform nature of the active sites has made possible a kinetic investigation of the mechanism of polymerization. Our evidence points to alkylchromium(IV) active centers, bound to silica by two Si-O-Cr linkages, which coordinate ethylene by displacement of a surface siloxane. Insertion of the coordinated olefin into one of two growing alkyl chains occurs provided there is no alpha -substituent. The information thus obtained may be used to understand the relationship between active site structure, catalyst activity and polymer properties.