Coordinatively unsaturated Cu(CN)(2)(-) forms mixed-ligand complexes, of the general formula Cu(CN)(2)X(2-), in the presence of halide ions in aqueous solution. The stepwise formation constants have values of 0.70 +/- 0.12, 1.92 +/- 0.08, and 16 +/- 4 M(-1) for X = Cl-, Br-, or I-, respectively. Moreover, the excited state formed by UV irradiation of the dicyano complex, i.e., *Cu(CN)(2)(-), also associates with halide ions to form a highly luminescent species identified as an exciplex, with about an order of magnitude higher stepwise formation constant (11 +/- 3, 27 +/- 4, and 430 +/- 40 for Cl-, Br-, and I-, respectively) than the ground-state reaction. The trend in formation constants demonstrates the increasing stability of the exciplex with polarizability of the halo ligand. Luminescence quenching studies lead to the assignment of the exciplex as a triplet species. Activation parameters for exciplex formation and decay, obtained from temperature dependence studies of luminescence lifetimes measured at varying halide ion concentration, are consistent with the assignment of the precursor and the exciplex moieties, and contribute to the elucidation of the dynamics of the system.