CW EPR, pulsed ENDOR, and one- and two-dimensional ESEEM techniques have been applied to study the influence of H3PO4 and H3BO3 acid modification of gamma-Al2O3 on the strong electron acceptor sites of gamma-Al2O3 using adsorbed anthraquinone and o-chloranil as probe molecules. After the adsorbtion, electron transfer produces free radicals at active sites on the activated gamma-Al2O3. The observed spectra are the superposition of spectra from radicals simply adsorbed on the surface and from three different paramagnetic complexes of anthraquinone with coordinatively unsaturated aluminum. The isotropic hyperfine coupling of aluminum nuclei in these complexes is similar to 2.5, 21, and 28 MHz. Treatment of gamma-Al2O3 with H3PO4 decreases the relative amounts of adsorbed radical and the complex with 2.5 MHz hyperfine coupling. The H3BO3 treatment, in contrast, gives only the complex with the 21 MHz hyperfine coupling, and its resolved EPR spectrum indicates two equivalent aluminums in this active site. The ESEEM spectra show only weak dipole-dipole interaction with P-31 and B-11 nuclei located around the paramagnetic complex. These results suggest that acid modification alters the catalytic properties by changing the number and type of catalytic sites on the surface without introducing new species.