B3LYP hybrid density functional theory method is employed to study the five carotenoid radical cations of canthaxanthin (1), 7',7'-dimethyl-7'-apo-beta-carotene (2), 8'-apo-beta-carotene-8'-al hydrazone (3), 7',7'-dicyano-7'-apo-beta-carotene (4), and 8'-apo-beta-carotene-8'-al (5). The radicals are characterized by means of their geometries, spin populations, and isotropic hyperfine coupling constants. It is shown that for all the systems, the unpaired spin is delocalized over the whole pi-conjugated system in an odd-alternant pattern. As a result of this, the hyperfine coupling constants are rather low. The radical cations of 1, 2, and 3, have very similar properties to the unsubstituted P-carotene radical, while the dicyano-and aldehyde-substitutions result in significantly different electronic structures. (C) 2002 Elsevier Science B.V. All rights reserved.