Using a high-activity MgCl2-supported TiCl4 catalyst, kinetic studies of ethylene and propylene polymerization are conducted in a semi-batch gas phase stirred-bed reactor system. Based on the experimental observations obtained from this study and others in the literature, simple kinetic mechanisms are proposed to explain the data. This model considers both the site formation from the interaction of catalyst and cocatalyst as well as the participation of monomers during site activation. By using this model together with parameters estimated from various sources, some aspects of kinetic behavior have been successfully predicted. These include the rate enhancement introduced by cu-olefins, the effect of the Al/Ti ratio on kinetic features such as catalyst activity and decay rate, as well as the different reaction orders observed for various monomers.