Complexation of the cadmium(II) ion with thiocyanate and iodide ions have been studied by precise titration calorimetry in micellar solutions of a nonionic surfactant Triton X-100 containing 0.4 mol dm-3 NaClO4 as a constant ionic medium at 298 K. With regard to the Cd(II)-SCN- system, the formation of [Cd(NCS)]+ and [Cd(NCS)(SCN)] in aqueous solution and [Cd(NCN)2(SCN)]- in micelles was established, though the complexation in micelles is not extensive. With the Cd(II)-I- system, the formation of [CdI]+, [CdI3]-, and [CdI4]2- was established in aqueous solution together with the formation of [CdI2], [CdI3]-, and [CdI4]2- in micelles. It is revealed that the complexation in micelles is accompanied by relatively large and negative enthalpies, which is ascribed to the enthalpies of transfer of [CdI3]- and [CdI4]2- from aqueous solution to micelles. This suggests that a hydrophobic complex with no capacity to form strong hydrogen bonds in water is favored in micelles.