Commercial gas-phase polymerization of ethylene is commonly carried out over Ti-based catalysts in which the Ti complex is supported on porous silica gel containing MgCl2. It is almost always assumed in kinetic and reactor modelling that the rate functions for the rate of polymerization over these catalyst systems are first order with respect to ethylene concentration. We have observed reaction orders with respect to ethylene from less than unity to about 1.5 for homopolymerization of ethylene in the gas-phase over SiO2/MgCl2 supported Ti catalysts with a tri-n-octylaluminum alkyl cocatalyst. Tri-n-octylaluminum alkyl was used to obtain relatively time-invariant rates of polymerization. Rates were measured with two catalysts at temperatures of 30 to 70 degrees C and ethylene concentrations of 15 to 750 mol/m(3). The reaction orders were found to be a strong function of temperature. Langmuir-Hinshelwood rate functions are used to describe the kinetics; a combination of at least two Langmuir-Hinshelwood rate functions is required to describe the observed rate behavior. This strongly suggests that at least two catalytic sites, having rates with different ethylene concentration dependencies, are present in these catalysts.