A novel laboratory slurry reactor useful for studies of polymerization of olefins with solid catalysts such as Cr/silica is described. The operation of this reactor, in which gaseous impurities are minimized by purification of incoming gases and by in situ activation of the catalyst, has enabled the early stages of the homopolymerization of ethylene to be studied without interference from catalyst deactivation effects. By standardizing the conditions for the experimental runs, the reactor demonstrated its usefulness in isolating and assessing the relative physical roles of different silica supports. Four catalysts supported on different silica supports were compared, and their differences in promoting the polymerization were interpreted in terms of porosity, average particle size, and changes in specific surface area. During early polymerization, the fracturing of catalysts prepared by impregnation of the porous silica support ensures access to these original Cr sites, in the absence of additional concealed sites. At low yields, the polymerization process is clearly affected by the physical character of the silica support; however, at higher yields, the evidence suggests that other causes such as chemical effects are responsible for the continuing acceleration in rate. The porous Cr/silica catalyst tested did not fracture instantaneously; hence, current models of nascent polymer do not describe this process. The "hardcore" model of nascent polymer seems applicable to polymerization on the nonporous Cr/Cab-O-Sil catalyst tested.