The crystal structure of tetrakis(tetrahydro-1H-pyrimidine-2-thione)palladium(II) dichloride monohydrate has been determined. X-irradiation of a single crystal, at liquid-nitrogen temperature, leads to formation of a paramagnetic species whose ESR study has been performed at 77 K. The g and ¹⁰⁵Pd-hyperfine tensors have been obtained. Their eigenvalues are consistent with the trapping of a d⁹ ion. Comparison of the eigenvectors with the crystallographic bond directions shows that the paramagnetic species results from the reduction of Pd(II) to Pd(I) which remains at the center of a square formed by four sulphur atoms.