The side-chain-side-chain interaction between Phe residues and sulfur-containing residues (Cis and Met) in the two possible orientations at positions i, i + 4 of alpha-helices is described. We have analyzed the contribution to helical stability of the above interactions by studying eight polyalanine-based peptides differing at the residues at positions 9 and 13. These two positions were independently mutated from Ala (AA), to Cys (AC and CA), Met (AM and MA), and Phe (AF and FA) and to the pairs Phe-Met (FM), Met-Phe (MF), Phe-Cys (FC), and Cys-Phe (CF). The intrinsic helical propensities of Cys, Met, and Phe were found to be those previously described in the algorithm AGADIR. NMR analysis of the FM, MF, FC, and CF peptides showed the formation in aqueous solution of contacts between the aromatic ring and the side chains of Cys or Met, at the two i, i + 4 orientations. CD studies demonstrated the important contribution of two of these interactions (FM and FC) to alpha-helix stability (up to 2 kcal mol-1 in the Phe-Cys pair). Statistical analysis of the protein database provides a rationale for the stereospecificity and free energies of the interactions. The very favorable interaction between an aromatic ring and a sulfur-containing amino acid explains why in the protein database around 50% of the sulfur atoms are contacting aromatic rings (Reid et al., 1985).