The coordination behaviour of the novel ligand, HMPz4Cy, is reported, together with solid state isolation of its diamagnetic cobalt(III) complexes, [Co(MPz4Cy)2]X·nH2O (X = Cl, Br, NO3, ClO4 and BF4). I.r. and 1H-n.m.r. data for the free ligand and its CoIII complexes confirm that the ligand, HMPz4Cy, acts as a uninegative anion with NNS tridentate function via the pyrazolyl nitrogen (tertiary), azomethine nitrogen and thiol sulphur. Electronic spectra (both solid and solution) are commensurate with a distorted octahedral environment for the reported CoIII species. Cyclic voltammograms of CoIIIcomplexes indicate a quasireversible Co+3/Co+2 couple. X-ray crystallography of a representative species, [Co(MPz4Cy)2]Cl·2.75H2O (C2, monoclinic), has shown unambiguously that the two ligands are orthogonally coordinated to the central CoIII ion with both the thiolato sulphurs and both pyrazolyl nitrogen atoms in cis positions.