An electron spin resonance (ESR) study of phosphorus centered electron capture radicals trapped in single crystals of substituted diphosphine disulfides [R2P(S)P(S)R2, R=Me, Et, Ph] irradiated with x rays at 77 K is reported. The principal values and direction cosines of the hyperfine and g tensors were determined. It is shown that in all three compounds a three-electron P-P bond radical is formed. The unpaired electron is found to be symmetrically distributed over the two phosphorus atoms. The direction of the phosphorus hyperfine coupling makes an angle of 20°-30° with the P-P bond. The alkyl substituted diphosphine disulfides reveal furthermore a radical in which the unpaired electron is asymmetrically distributed over the two phosphorus atoms. The electronic configuration of these radicals is best described as trigonal bipyramidal (TBP) with a central phosphorus atom and the unpaired electron in an equatorial position (TBP-e). The second phosphorus atom and a sulfur substituent are located on the apical sites of the TBP. Irradiation yields for R=Et, Ph also a three-electron P-S bond radical and a number of dissociation products. Ab initio quantum chemical calculations of the isotropic and anisotropic hyperfine couplings for the three-electron P-P bond radicals are in good agreement with the experimental values and support the assignment.