The e.s.r. spectra of the copper(II) chelates of L-β-(3,4-dihydroxyphenyl)alanine (L-dopa), α-methyldopa, 3,4-dimethoxy-α-methyldopa, dopamine, 3,4-dihydroxyphenylacetic acid, adrenaline, noradrenaline, and catechol formed in dimethylformamide (dmf) or dimethyl sulphoxide (dmso) have been studied. At room temperature the results are consistent with the formation of monomeric chelates though in frozen solution the existence of dimeric species has been shown. Computer simulation of the spectra of the dimeric species has been used to propose structures for the dimeric chelates and the influence of the functional groups in the ligand has been assessed by comparing similar results for the chelates formed with the related series of ligands.