A large single crystal of bis(ethy1enediamine)platinum bis(hydrogen squarate) ( [Pt(en)z](HSq)2), has been X-irradiated at 77 K and studied by EPR. The g and I 9 5 P t hyperfine tensors obtained from the angular dependence of the spectrum are consistent with the trapping of a Pt(II1) complex containing the unpaired electron in a dz2 orbital. Comparison of the orientations of the eigenvectors with those of the crystallographic interatomic vectors together with the predictions of Xa calculations on the model complex [Pt(NH3)4(0H)2]+ leads to a mechanism whereby a radiogenic radical, formed by homolytic scission of the hydrogen squarate hydroxyl, oxidizes the Pt(en)2 dication and yields [Pt(en)l(Sq)] (HSq).