The well-resolved ESR powder spectra of blue platinum complexes prepared from 1-methylhydantoin and 5-methyl-2-pyrrolidinone offer the possibility of probing various structural models. A good agreement between the experimental and the calculated spectra is obtained by assuming that the hyperfine structure is attributed to the coupling of an unpaired electron with four ¹⁹⁵Pt nuclei involved in a tetrameric structure.