The equivalent conductivities of tris-(ethylenediamine)cobalt(III) complexes, [Co(en)(3)]X-3 (X = Cl-, Br-, I-; en = ethylenediamine) were measured as functions of temperature (288.15-328.15 T/K) and concentration (1.958 x 10(-4)-10.260 x 10(-4) c/mol.dm(-3)) in dimethyl sulfoxide (DMSO). Equivalent conductivity values for [Co(en)(3)]X-3 in DMSO were varied in the order Cl- > Br- > I-. The conductivity data were analyzed by using Robinson-Stokes equations. For [Co(en)(3)]X-3, the limiting equivalent ionic conductivities of [Co(en)(3)](3+) and the ion association constants (K-A) of the ion pair formed between [Co(en)(3)](3+) with monovalent halide anions were determined in DMSO. K-A values for three complex salts in DMSO were decreased in the order Cl- > Br- > I-. While K-A values for [Co(en)(3)]Cl-3 apparently increased with increase in temperature, those of [Co(en)(3)]Br-3 and [Co(en)(3)]I-3 slightly increased with temperature. The temperature variations of the association constants for [Co(en)(3)]X-3 in DMSO have also been studied to get the thermodynamic functions (standard Gibbs energy, enthalpy and entropy changes). (C) 2013 Elsevier B.V. All rights reserved.