Mobile-phase Cu(His)2 complexes in water at room temperature in the presence of excess histidine are characterized by ESR methods. In order to identify the nitrogen donor atom, histidine, for which I4N in the imidazole ring is substituted with I5N, was used. Spectra in the presence of excess histidine have been analyzed by simulation of second-derivative spectra and by Fourier and reverse Fourier transforms of the spectra. It is concluded that the spectra arise from a superposition of an 80% contribution from complexes formed with two histidines in a histamine-like configuration and a 20% contribution from complexes with two histidines: one glycine-like and one histamine-like. It is suggested that Fourier and reverse Fourier transforms are particularly effective methods to analyze the nitrogen superhyperfine patterns in spectra from copper-histidine complexes.