A thiol-thioester exchange system has been used to measure the propensities of diverse beta-amino acid residues to participate in an alpha-helix-like conformation. These measurements depend on formation of a parallel coiled-coil tertiary structure when two peptide segments become linked by thioester formation. One peptide segment contains a "guest" site that accommodates diverse beta residues and is distal to the coiled-coil interface. We find that helix propensity is influenced by side chain placement within the beta residue [beta(3) (side chain adjacent to nitrogen) slightly favored relative to beta(2) (side chain adjacent to carbonyl)]. The previously recognized helix stabilization resulting from five-membered ring incorporation is quantified. These results are significant because so few quantitative thermodynamic measurements have been reported for alpha/beta-peptide folding.