Ethylene was polymerized on CP2ZrCl2 activated by methylalumoxane (MAO) supported on boehmites modified with arylsulfonic acids. In comparison with unmodified boehmites, boehmites modified with arylsulfonic acids gave significantly higher MAO uptakes. Cp2ZrCl2 activated with arylsulfonic acid-modified bochmites/MAO gave notably higher catalyst activities compared with those of homogeneous CP2ZrCl2/MAO and CP2ZrCl2 activated with unmodified boehmite/MAO and SiO2/MAO. According to C-13 CP-MAS NMR investigations of MAO supported on p-toluenesulfonic acid-modified boehmite, partial alkylation of p-toluenesulfonate by MAO accounted for the formation of methyl p-tolyl sulfone as an electron-donating catalyst modifier. Polymer particle morphologies and size distributions were examined off-line by means of scanning electron microscopy, light scattering on dry powder dispersions, and CCD-mediated powder particle image analysis. The mechanism of polymer particle formation via deagglomeration of boehmite particles was elucidated by means of on-line monitoring using focused beam reflectance measurement. As evidenced by transmission electron microscopy, polyethylene particles contained uniformly dispersed boehmite crystallite aggregates with average diameters of 100 nm.