Lewis acidic sites (LAS) of silica, modified with TMA and MAO samples differed by TMA content, have been characterized by IR spectroscopy (CO adsorption as probe molecule at 77 K). Two types of LAS were found on the surface of silica modified with MAO and TMA :M LAS of moderate strength(v(CO) = 2204-2212 cm(-1)) and weak W LAS (v(CO) = 2194 cm(-1)). The concentration of these acidic sites has been estimated. It was shown by IRS study Cp2ZrMe2 interacts both with W LAS and M LAS. Correlation between the amount of M LAS and the activity of ethylene polymerization has been found.