The decay rate constants of the photogenerated biradical of a phenothiazine-C-60 linked compound dramatically decreased in lower magnetic fields (0-0.2 T) and then gradually increased in higher magnetic fields (0.2-1 T) in benzonitrile at various temperatures (283-343 K). As temperature increased, the magnitudes of magnetic field effects (MFEs) in the lower magnetic fields were reduced, while those in the higher magnetic fields were enhanced. These reverse phenomena in the MFEs are explained by the contribution of spin-lattice relaxation mechanism due to anisotropic Zeeman interaction. The temperature effect supports the above mechanism and is probably ascribed to the molecular motion in the biradical. (C) 2004 Elsevier B.V. All rights reserved.