The presence of simultaneously existing multiple species were taken into account in interpreting the structures of the complex species in the Cu(II)–L-histidine system. A refined proton displacement technique was utilized for structure elucidation of the species in solution. The absorbance and the molar extinction of the individual species were computed from the total absorbance and the species distribution over the pH range studied. The i.r. spectra of L-histidine and Cu(II)–L-histidine in D2O were obtained as a function of pD. They were interpreted in view of the species detected in a given pH range. The results of the species distribution as a function of pH, proton displacement, visible spectra of the individual species, and infrared data, were all combined to make structural interpretations. The following modes of metal coordination were proposed for the major species in the Cu(II)–L-histidine system: MHA, O(carboxyl) and N(imidazole); MH2A2, O(carboxyl) and N(amino) in both L-histidines; MA, O(carboxyl), N(imidazole), and N(amino); MHA2O(carboxyl), N(imidazole), and N(amino) in one L-histidine and O(carboxyl) and N(amino) in the other; MA2, O(carboxyl). N(imidazole), and N(amino) in both L-histidines, or O(carboxyl), N(imidazole), and N(amino) in one L-histidine and N(amino) and N(imidazole) in the other or an equilibrium mixture of both structures.