The polymerization of propylene in the gas phase has been studied with the same high-activity catalyst as was used for liquid-phase polymerizations in earlier work. Catalyst injection, the influence of the support bed, and precontacting of the catalyst with the cocatalyst and the electron donor have been investigated. Furthermore, different ways of prepolymerization were tested. The kinetics can be described with the same model as was used for the liquid-phase polymerizations. The reaction rate was proportional to the pressure at pressures above 5 bar; at lower pressures the rates are lower. Comparison of the reaction rates in the gas phase and liquid phase showed the rates in the gas phase were still lower than in the liquid phase. The fast decay observed in the gas-phase polymerizations indicated that thermal runaway and deactivation may occur at temperatures above 40–45°C.