For a determination of the active site concentrations, [C*], the simultaneous kinetics and inhibition method using CO as a catalyst inhibitor were examined. It is noticed that this evaluation method is not quantitatively exact because MAO-activated alkyl Zr sites exist in dynamic equilibrium between dormant states and a chain propagating site and because some CO molecules coordinate even to inactive species. The number of active sites was evaluated on the assumption that two molecules of CO coordinate to each active center. The increase in Al/Zr ratio and temperature increased both the number of active sites and the value of k(p) for ethylene polymerization over Cp(2)ZrCl(2)/MAO. Especially, a drastic increase of activity is mainly due to a much higher value of k(p) above a Al/Zr ratio of 2000. A comonomer enhanced drastically the catalytic activity to more than 10 times for ethylene-propylene copolymerization over the rac-Me(2)Si(Ind)(2)Zrcl(2)/MAO system. This is due to the drastic increase in the propagating rate constant, k(p), rather than the increase in the formation of new active sites by the addition of comonomer.