Determination of the tendencies of amino acids to form α-helical and β-sheet structures has been important in clarifying stabilizing interactions, protein design, and the protein folding problem. In this study, we have determined for the first time a complete scale of amino acid propensities for another important protein motif:  the collagen triple-helix conformation with its Gly-X-Y repeating sequence. Guest triplets of the form Gly-X-Hyp and Gly-Pro-Y are used to quantitate the conformational propensities of all 20 amino acids for the X and Y positions in the context of a (Gly-Pro-Hyp)₈ host peptide. The rankings for both the X and Y positions show the highly stabilizing nature of imino acids and the destabilizing effects of Gly and aromatic residues. Many residues show differing propensities in the X versus Y position, related to the nonequivalence of these positions in terms of interchain interactions and solvent exposure. The propensity of amino acids to adopt a polyproline II-like conformation plays a role in their triple-helix rankings, as shown by a moderate correlation of triple-helix propensity with frequency of occurrence in polyproline II-like regions. The high propensity of ionizable residues in the X position suggests the importance of interchain hydrogen bonding directly or through water to backbone carbonyls or hydroxyprolines. The low propensity of side chains with branching at the C^δ in the Y position supports models suggesting these groups block solvent access to backbone CO groups. These data provide a first step in defining sequence-dependent variations in local triple-helix stability and binding, and are important for a general understanding of side chain interactions in all proteins.