The crystal structure of the title phosphated sugar (I) has been determined. The angular variations of the e.s.r. spectra obtained with a single crystal of (I) X-irradiated at T 100 K have been analysed. They show that a radical pair is produced by irradiation and that a component of this pair is a phosphoryl radical. The orientations of the e.s.r. tensors are compared with the crystallographic bond directions and the role of the irradiation conditions on pair formation is examined. It appears that the phosphoryl radical is probably produced by electron capture by the cationic species [(RO)3PO]+.