Canthaxanthin and 8 ' -apo-beta -caroten-8 ' -al radical cations chemically prepared on activated silica-alumina and in CH2Cl2 solution containing AlCl3 were studied by pulsed EPR and ENDOR spectroscopies. Both the 1D three-pulse ESEEM and the 2D HYSCORE spectra of the carotenoid-AlCl3 mixtures exhibited the Al-27 nuclei peak at 3.75 MHz. This indicates electron-transfer interactions between carotenoids and Al-III ions resulting in the formation and stabilization of carotenoid radical cations. Davies ENDOR measurements of the canthaxanthin radical cation on silica-alumina determined the hyperfine couplings of protons belonging to three different methyl groups with a(H1) = 2.6 MHz, a(H2) = 8.6 MHz, and a(H3) ca. 13 MHz. The principal components of the proton hyperfine tensors were obtained from HYSCORE spectra in AlCl3 solutions and on the solid support. Identification of the protons was made on the basis of isotropic hyperfine couplings determined by RHF-INDO/SP molecular orbital calculations. In frozen AlCl3 solution, the C(7,7 ')-H alpha and C(14,14 ')-H alpha alpha protons were observed for canthaxanthin and the C(8 or 14 ')-H alpha and C(15 ')-H alpha were observed for 8 ' -apo-beta -caroten-8 ' -al. On the silica-alumina support, the C(10,10 ')-H alpha, C(11,11 ')-H alpha, and C(15,15 ')-H alpha alpha protons were measured for canthaxanthin and the C(12)-H alpha and C(15 ')-H alpha were measured for 8 ' -apo- beta -caroten-8'-al. Some protons with large isotropic couplings (> 10 MHz) determined from HYSCORE analysis could be assigned to beta protons, but the principal components of their hyperfine tensors are much more anisotropic than those reported previously for protons. We suggest that cis/trans isomerization of carotenoids on silica-alumina results in stabilization of di-cis isomers with large isotropic couplings for some alpha protons which are comparable to those of beta protons. HYSCORE is a promising technique to increase spectral resolution for proper assignment of protons with large hyperfine anisotropy, which cannot be resolved by ENDOR spectroscopy.