Cu-63 and Cu-65 nuclear magnetic resonance (NMR) and viscosity studies of 0.064 M CuClO4 have been made in binary mixtures of acetonitrile (AN) with succinonitrile (SN), adiponitrile (ADN), n-butyronitrile (n-BTN) and iso-butyronitrile (iso-BTN) as co-solvents at 298 K using 500 MHz NMR spectrometer and an Ubbelohde viscometer, respectively. Chemical shift (8) and linewidth (Delta) for both Cu-63 and Cu-65 signals have been recorded relative to 0.064 M CuClO4 solution in pure anhydrous AN. Quadrupole relaxation rates (1/T-2)(Q), reorientational correlation times (tau(R)) and copper quadrupole coupling constants (e(2)Qq/h) have been calculated in all cases. The variation of these NMR parameters has been examined as a function of molecular percent co-solvent to compare the relative effects of these co-solvents on the solvation behaviour of Cu+ and hence on the stabilization of copper (I) complexes formed. The (e(2)Qq/h) values in all AN+ nitrile mixtures are larger than the values in pure AN and increase with the increase of molecular percent co-solvent. The strong increase of (e(2)Qq/h) values in all AN+ nitrile mixtures with the increase of molecular percent co-solvent indicates the formation of mixed complexes of the form [Cu(CH3CN)(4-x)(S)(x)](+) (x = 1 - 4) due to the replacement of AN molecules from the original complex [Cu(CH3CN)(4)](+) formed in pure AN by the co-solvent molecules (S). The results also show that mixed copper (I) complexes in all cases remain stable even at high co-solvent compositions. The (e(2)Qq/h) values also indicate that the effect of dinitriles is relatively stronger than that of mononitriles, the effect of Cu-63 isotope is relatively stronger than that of Cu-65 and the effect of iso-BTN is relatively stronger than that of n-BTN in the formation of mixed copper (I) complexes. (C) 2003 Elsevier B.V. All rights reserved.