Abstract: Polymerization of olefin monomers is conducted using at least one d- or f-block metal-containing olefin polymerization catalyst compound or complex and a novel methylaluminoxane composition (MAOC) which is a solid at 25° C. that has a total aluminum content of about 39 to 47 wt %. The MAOC is either free of aluminum as trimethylaluminum (TMA) or if TMA is present, not more than about 30 mole % of the total aluminum in the MAOC is TMA. In the solid state the MAOC contains no more than about 7500 ppm (wt/wt) of aromatic hydrocarbon. The cryoscopic number average molecular weight of MAOC as determined in benzene is at least about 1000 amu, and the MAOC has sufficient solubility in n-heptane at 25° C. to provide a solution containing 4 to as high as 7.5 wt % or more of dissolved aluminum. By vacuum distilling a solution of ordinary MAO in aromatic hydrocarbon long enough under proper conditions, MOAC is formed.