Propylene was polymerized in a slurry phase over superactive and stereospecific catalyst prepared by the reaction of Mg(OEt)2 with benzoyl chloride and TiCl4 in the presence of AlEt3 with or without an external donor. A kinetic analysis of propylene polymerization was carried out. The polymerization rate was first order with respect to monomer concentration and the dependence of overall polymerization rate on the concentration of AlEt3 can be explained by the Langmuir adsorption mechanism. Maximum activity was observed around an Al/Ti mole ratio of 20. The average rate over 90 min of polymerization as a function of temperature showed a maximum around 42-degrees-C and the overall activation energy was 8.5 kcal/mol at T < 42-degrees-C and -4.0 kcal/mol at T > 42-degrees-C. The analysis of the phenomenon of an optimum temperature gave 2.2 kcal/mol for the activation energy of the rate-determining step, and 6.3 kcal/mol, for the adsorption energy of AlEt3. The addition of small amount of p-ethoxyethyl benzoate (PEEB) as an external donor increased the percentage of isotactic polymer to 98% and slightly increased activity in spite of the decrease in the concentration of active centers due to the stabilizing effect of the active centers by the external donor. The temperature showing maximum yield was shifted to the higher temperature when AlEt3 and PEEB ([AlEt3]/[PEEB] = 5) was used as a cocatalyst.