The radicals in an irradiated single crystal of α-glycerol phosphate have been studied by ESR spectroscopy. Two distinctive doublets have been observed. One, characterized by g1=1.9974, g2=2.0216, g3=2.1074 and A1=−2.61, A2=−4.30, A3=0.42 mT, is attributed to the trapped hydroxyl radical. The other, with the g-tensor elements g1=2.0375, g2=2.0063, and g3=2.0063 and the hyperfine couplings A1=2.84, A2=3.00, and A3=2.78 mT, is assigned to the PO2−4 radical. The distinction between the PO2−4 radical and the alkoxy radical has been made on the basis of the mutual orientations of the g and A tensors.