Formation of cobalt(II)-thiocyanato complexes in nonionic surfactant solutions of poly(ethylene oxide) type with varying poly(ethylene oxide) chain lengths of 7.5 (Triton X-114), 30 (Triton X-305), and 40 (Triton X-405) has been studied by titration spectrophotometry and calorimetry at 298 K. Data were analyzed by assuming formation of a series of ternary complexes Co(NCS)nYm (2-n)+ (Y = surfactant) with an overall formation constant βnm. In all the surfactant systems examined, data obtained can be explained well in terms of formation of Co(NCS)+ and Co(NCS)2 in an aqueous phase (aq), and Co(NCS)4Y2- in micelles, and their formation constants, enthalpies, and entropies have been determined. The β41/β20 ratio increases and the corresponding enthalpy becomes significantly less negative with an increasing number of ethylene oxide groups. This suggests that micelles of these nonionic surfactants have a heterogeneous inner structure consisting of ethylene oxide and octylphenyl moieties. Indeed, on the basis of molar volumes of ethylene oxide and octylphenyl groups, intrinsic thermodynamic parameters have been extracted for the reaction Co(NCS)2(aq) + 2NCS-(aq) = Co(NCS)4Y2- (ΔrG°, ΔrH°, and ΔrS°) at each moiety. The ΔrG°, ΔrH°, and ΔrS° values are -16 kJ mol-1, -15 kJ mol-1, and 3 J K-1 mol-1, respectively, for the ethylene oxide moiety, and -15 kJ mol-1, -70 kJ mol-1, and -183 J K-1 mol-1 for octylphenyl. Significantly less negative ΔrH° and ΔrS° values for ethylene oxide imply that the hydrogen-bonded network structure of water is extensively formed at the ethylene oxide moiety, and the structure is thus broken around the Co(NCS)4 2- complex with weak hydrogen-bonding ability. © 2001 Academic Press.