A systematic study has been made on the functions of external Lewis base (Be, methyl-p-toluate, MPT) and internal Lewis base (Bi, ethyl benzoate, EB) for the CW-catalyst system MgCl,/EB/PC/AlEt,/TiCl,-AlEt,/MF'T (PC, p-cresol). Bi is a nonstereoselective modifier. It increases the active site concentrations and rate constants of propagation, k,, of both the isospecific and nonspecific sites, and thus the productivities of the stereoregular and irregular polypropylenes by five-to tenfold. It seems that Bi complexes with the MgC1, support to lower the electronegativity of the surface Mg atoms. It also acts to lower the rate constant of chain transfer to aluminum alkyl, ktf, by two-to fourfold. The action of Be is highly stereospecific. The isotacticity index of polypropylene is 2 95% in the presence of Be but s 68% without it. Addition of Be decreases nonspecific [Ti*], by about (11 & 2)-fold; there is only about a twofold reduction of the isospecific [TiIi. It decreases kP,= about three times but has no effect on k,,i, so that the latter is (21 f 4) times the former. Be decreases k;",., for transfer with aluminum alkyl much more than it does to ktf,,; but it does not affect the rates of chain transfer with monomer and by /3-hydride elimination or the rate of catalyst deactivation. The number of active sites without Be is [TiIi = 15% and [Ti*], = 55% for a total of 70%. In the presence of Be they are both about 6%. Optimum performance in propylene polymerizations requires both B, and Be in the case of the CW-catalyst.