The effects of pH, temperature, and stoicheiometry on e.s.r. spectra of the copper(II)–carnosine complex have been measured at both X- and S-band. Four different complexes of Cu2+ and carnosine are found. At physiological pH a magnetically coupled copper(II) dimer is formed, but this is converted into a monomer in which the Cu2+ is co-ordinated to four equivalent carnosine molecules when the carnosine is present in excess. Anserine produces the same complexes as does carnosine, but homocarnosine and glycyl-L-histidine produce only the monomeric complex. The mechanism of crystallization of copper–carnosine and its possible importance in biological systems is reappraised.