Binding affinities of tryptophan dipeptides, Trp-X(X= A, F, G, I, K, L, S, T, V, W, and Y),X-Trp (X= A, F, G, K, L, P, R, V, W, and Y), andX-Trp-NH2(X= A, F, G, L, Q, S, V, and W) types to sodium dodecyl sulfate (SDS) micelles were studied using the conventional method of ultraviolet absorption spectrophotometry. The free energies of transfer for the dipeptides from the micellar to the aqueous phases were obtained from the values of the distribution coefficients estimated on the basis of the ratio of absorbances at the peaks around 280 nm to those at the valleys around 245 nm. The free energies depend on the positive and negative charges and also on the hydrophobicity of the amino acid residues of the dipeptides. It was observed that the hydrophobic amino acid residues stabilized the complexes corresponding to the free energies of transfer for amino acid residues of peptides from octanol to water and that small hydrophobic residues, such as Ala, did not contribute to the affinities of the dipeptides for the SDS micelles. The large values for the effective dielectric constant,Deff, suggest that the indole rings of the dipeptides exist in and/or near the interfaces of the SDS micelles. These results suggest that the dipeptide backbone in the dipeptide-SDS micelle complex exists in the hydrophilic area of the micelle and that a part of the hydrophobic side chain is immersed in the hydrophobic area of the micelle. It was also observed for charged dipeptides that the free energies depended on their amino acid sequences.