Abstract: The ESR spectra of TEMPONE radical on the activated Al₂O₃ were studied. It is found out that the ESR spectra of well-resolved nitrogen hyperfine splittings from TEMPONE adsorbed at two distinct types of sites can only be obtained when the Al2O₃ must be treated previously with the steam at elevated temperature to increase the adsorbed hydroxyl group on the Al₂O₃ surface. The nitrogen hyperfine constant AN increases with the increasing of Lewis acid strength.Thus it may be used as a measure of the strength of the surface acid.