Complexation of copper(II) and zinc(II) with thiocyanate ions has been studied by precise titration spectrophotometry and calorimetry in aqueous solution containing varying concentrations (0-100 mmol dm-3) of a non-ionic surfactant Triton X-100 at 25°C. Both spectrophotometric and calorimetric data were analysed by treating the surfactant as the third component, and the complexes formed in aqueous and micellar pseudo phases were distinguished. In the case of zinc(II), the formation of [Zn(NCS)]+ and [Zn(NCS)3]- in the aqueous phase and [Zn(NCS)3Y]- and [Zn(NCS)4Y]2- (Y = Triton X-100) in the micellar phase was established, and their formation constants, enthalpies and entropies obtained. On the other hand, with copper(II), the formation of [Cu(NCS)]+, [Cu(NCS)2] and [Cu(NCS)2Y] was established, and their formation constants and intrinsic electronic spectra were extracted. As the complexation behaviour in a micellar solution might be compared with that in a non-aqueous solution, the complexation for the CuII-NCS- system was also studied in N,N-dimethylformamide. The difference in the complexation of copper(II) and zinc(II) ions in the micellar solution is discussed from thermodynamic and structural viewpoints.