X-irradiation of single crystals of K2[Pt(NO2)] and of K2[Pt(NO2)4]·2H2O leads to the formation of Pt(III) complexes which have been studied by EPR at 77 K. The crystal structure of the hydrated compound has been determined in order to interpret the orientation of the g and 195Pt hyperfine tensors and to reassign the powder spectrum previously attributed to this compound. The nature of the trapped species is shown to be dependent upon the presence of water molecules: one of the two Pt(III) complexes formed in the hydrated crystal probably results from oxidation of the original molecule by the radiogenic OH· radical whereas, for the second species, the water molecule seems only to participate in the stabilization of the Pt(III) complex. For the anhydrous compound this stabilizing effect is probably assured by a distant NO2 group.