The spectroscopic characteristics of the isostructural compounds [M(aat) 2(H 2O) 2]X 2 (aat = 3-acetylamino-1,2,4-triazole; M = Co, Cu, X = Br, CI; M = Ni, X = Cl) are described. The structure of [Co(aat) 2(H 2O) 2]Br 2 has been determined by X-ray diffraction methods. Crystals are monoclinic, space group P2 1/ a with a = 9.686(3), b = 14.433(4), c = 6.169(2) Å, β = 92.84(2)° and Z = 2. The structure was refined to final R and R′ values of 0.033 and 0.039 for 1066 observed reflections. In the mononuclear cationic complex, having an imposed crystallographic C i symmetry, the cobalt atom is surrounded in a slightly distorted octahedral arrangement by two axial water molecules (CoOw = 2.108(5) Å) and in the equatorial plane by two oxygen atoms (CoO(1) = 2.100(4) Å) and two triazole nitrogen atoms (CoN(3) = 2.063(5) Å) from two aat chelating ligands. The complex cations are associated in chains, with interposed bromide anions, by hydrogen bonding system.