A systematic investigation of the CuCl 2/Mebta (Mebta = 1-methylbenzotriazole) reaction system is described, involving the determination of the influence of the Cu II:Mebta ratio, the nature of solvent and the presence of counterions on the identity of the reaction products. As a consequence, complexes [Cu 2Cl 4(Mebta) 4] ( 1), [CuCl 2(Mebta) 2] ( 2), {[Cu 2Cl 4(Mebta) 2]} n ( 3), [Cu 4OCl 6(Mebta) 4] · 0.25H 2O ( 4 · 0.25H 2O) and [Cu 2Cl 2(Mebta) 6](ClO 4) 2 ( 5) have been isolated and structurally characterized by single-crystal X-ray studies. Mebta behaves as a monodentate ligand binding through N(3). 1 is a dinuclear complex, the structure of 2 consists of discrete monomeric units, and that of 3 is composed of linear, well-separated polymeric chains of Cu II atoms. The molecules of 4 · 0.25H 2O have a central μ 4-oxide ion surrounded tetrahedrally by four Cu II atoms. In the cations of 5 the two Cu II centres are asymmetrically bridged by two chloro ligands, with three Mebta molecules completing five coordination at each metal. Complexes were characterized by spectroscopic (IR, far-IR, solution UV/Vis) and thermal decomposition (TG, DTG, and DTA) techniques. Variable-temperature magnetic susceptibility data for 1, 3 and 5 showed intramolecular ( 1, 5) and intrachain ( 3) ferromagnetic exchange interactions. Estimates of the Jparameters, experimentally derived, were in close agreement with a new magneto-structural criterion developed by us, holding for bis(μ-chloro) copper(II) dimers. A comparison between the CuCl 2/Mebta and CuBr 2/Mebta systems is also presented.