Three types of methylaluminoxanes (MAO-1, 2 and 3) were synthesized with different average molecular weights. In each MAO synthesis, two types of MAOs were isolated (Fractions A and B). Fractions A and B differ by the presence of different methyl/aluminum molar ratio and mol% trimethylaluminum. Kinetics of ethylene polymerization were studied using both fractions A and B of MAOs 1,2 and 3 as cocatalysts and Cp(2)ZrCl(2) as catalyst at atmospheric pressure of ethylene. The poly(ethylene)s were analyzed for their molecular weight.