Single crystals of cytosine hydrochloride with thiocytosine as an impurity were found suitable for the study of a possible new mechanism of long-range migration of energy deposited by ionizing radiation. In a crystal containing thiocytosine, two kinds of chlorine-containing paramagnetic centres are present that are completely absent in the pure cytosine. HCl crystals irradiated under the same condition. The thiocytosine molecules in conjunction with Cl- ions behave as hole traps. The centres have been characterized by EPR spectroscopy. One of the centres is derived from the cationic thiocytosine radical by interaction with a Cl- ion, and the other centre is formed by interaction of Cl. with a thiocytosine molecule. It is suggested that the transfer of an electron-loss site (a hole) in the Cl- network is the actual mechanism of the long-range energy transfer.