Formation of copper(II) thiocyanato and cadminum(II) iodo complexes in micelles of poly(ethylene oxide) (PEO)-type nonionic surfactants with varying PEO chain lengths of 9.5 (Triton X-100), 30 (Triton X-305), and 40 (Triton X-405) has been studied by titration spectrophotometry and calorimetry at 298 K. In a given surfactant solution, all data obtained were analyzed by assuming formation of ternary complexes MXnYm(2-n)+ (M = Cu-II,Cd-II; X =SCN-, I-; Y = surfactant), and the complexes thus form in aqueous phase (m = 0) or in micelles (m = 1). In the (CuSCN-)-S-II system, spectrophotometric data obtained by varying concentrations of the surfactant can be explained well in terms of formation of Cu(NCS)(2)Y in micelles and Cu(NCS)(+) and Cu(NCS)(2) in an aqueous phase, and it turned out that formation constant of Cu(NCS)(2)Y increases with increasing PEO chain length. In the Cd-II-I- system, the formation of CdI3Y- and CdI4Y2- is concluded in micelles, and that of CdI+, CdI3-, and CdI42- in an aqueous phase. Interestingly, formation enthalpies of CdI3Y- and CdI4Y2-become significantly less negative with increasing PEO chain length. This suggests that transfer of the complexes from aqueous solution to a hydrophobic octylphenyl (OP) moiety in micelles is significantly more exothermic than that to a hydrophilic PEO one. Thermodynamic parameters of transfer of CdI3- and CdI42- from aqueous solution to the OP and PEO moieties of micelles have been evaluated,