The tendency of histidine to form biologically important complexes with transition metal ions is well known. However, in the literature there is little information about the thermochemical behaviour of these complexes in the solid state, and the results are often contradictory. In this work, the synthesis and the thermoanalytical investigation of complexes of formula MeHis(2), MeHis(2)(NO3)(2) and MeHis(2)(NO3)(2) are reported, with Me = Co(II), Ni(II) or Cu(II) and His = histidine. The results show anomalous behaviour for histidine as a ligand compared with imidazole and its derivatives, in metal-ligand-nitrate ion interactions.