The isomeric trans-1 and cis-1 radicals have been studied by EPR spectroscopy. At 220 K, the EPR spectrum of the trans-1 isomer shows a hyperfine structure corresponding to the coupling of the unpaired electron with the different nuclei with non-zero magnetic moments. In contrast, the EPR spectrum of the cis-1 isomer at similar temperatures shows the characteristic structure of a triplet species. The differences in the EPR spectra of both isomers have been attributed to the fact that the cis-1 isomer aggregates in solution originating the thermodynamically stabilized hydrogen-bonded diradical species, in which strong antiferromagnetic interactions are developed. EPR frozen solution experiments, down to liquid helium temperature, showed that the magnetic ground state of the resulting dimeric species is the singlet state and the triplet should be associated with a thermally accessible excited state.