N-Methylacetamide, a model of the peptide unit in proteins, is allowed to interact with CH3SH, CH3SCH3, and CH3SSCH3 as models of S-containing amino acid residues. All of the minima are located on the ab initio potential energy surface of each heterodimer. Analysis of the forces holding each complex together identifies a variety of different attractive forces, including SH⋅⋅⋅O, NH⋅⋅⋅S, CH⋅⋅⋅O, CH⋅⋅⋅S, SH⋅⋅⋅π, and CH⋅⋅⋅π H-bonds. Other contributing noncovalent bonds involve charge transfer into σ* and π* antibonds. Whereas some of the H-bonds are strong enough that they represent the sole attractive force in several dimers, albeit not usually in the global minimum, charge-transfer-type noncovalent bonds play only a supporting role. The majority of dimers are bound by a collection of several of these attractive interactions. The SH⋅⋅⋅O and NH⋅⋅⋅S H-bonds are of comparable strength, followed by CH⋅⋅⋅O and CH⋅⋅⋅S.