Considering the interaction between excited triplet molecule and doublet radical, based on the second-order perturbation theory and the motion equation of density matrix, the spin polarization intensity on the radical was theoretically calculated. The results of calculation indicate that the polarization was hyperfine related. With the overpopulation on doublet spin states of radical-triplet pairs as initial condition, the CIDEP on the radicals were A + A/E pattern and with the overpopulation on quartet spin states of radical-triplet pairs, the CIDEP was E + E/A pattern. (C) 2003 Elsevier Science B.V. All rights reserved.