Single crystals of the disodium and dipotassium salt of glucose-1-phosphate, X-irradiated at 80 K or at 280 K, show the presence of PO3(2-) radicals at 295 K, formed by scission of the phosphate-ester bond at the phosphate side. The 31P hyperfine coupling constants were measured using X- and Q-band EPR spectroscopy. Typical values for these coupling constants are a magnitude of = 68 mT and a perpendicular = 53 mT. The g values were almost isotropic, slightly smaller than that of the free electron spin (g = 2.0023). There is no substantial reorientation of the phosphate group in the crystalline lattice upon radical formation. Directly after irradiation at 77 K the PO3(2-) radicals are not present, but their characteristic resonance grows in upon thermal annealing of the crystals. The radicals are probably formed from a carbon-centered radical precursor by secondary reactions resulting in the loss of the phosphate group, leaving a (diamagnetic) modified carbohydrate molecule behind. The alternative process of reductive cleavage of the phosphate-ester bond by electrons released from traps in the crystal upon thermal annealing is considered less likely. A second phosphate-centered species, with a magnitude of about 21 mT and a perpendicular about 15 mT, was detected in the dipotassium salt of glucose-1-phosphate only. Possible structures for this species are discussed.