The capped alpha/gamma-peptide foldamers Ac-gamma(ACHC)-Ala-NH-benzyl (gamma alpha) and Ac-Ala-gamma(ACHC)-NH-benzyl (alpha gamma) were studied in the gas phase under jet-cooled conditions using single-conformation spectroscopy. These molecules serve as models for local segments of larger heterogeneous 1:1 alpha/gamma-peptides that have recently been synthesized and shown to form a 12-helix composed of repeating C12 H-bonded rings both in crystalline form and in solution [Guo, L.; et al. J. Am. Chem. Soc. 2009, 131, 16018]. The gamma alpha and alpha gamma peptide subunits are structurally constrained at the C beta-C gamma bond of the gamma-residue with a cis-cyclohexyl ring and by an ethyl group at the C alpha position. These triamides are the minimum length necessary for the formation of the C12 H-bond. Resonant two-photon ionization (R2PI) provides ultraviolet spectra that have contributions from all conformational isomers, while IR-UV hole-burning (IR-UV HB) and resonant ion-dip infrared (RIDIR) spectroscopies are used to record single-conformation UV and IR spectra, respectively. Four and six conformers are identified in the R2PI spectra of the gamma alpha and alpha gamma peptides, respectively. RIDIR spectra in the NH stretch, amide I (C=O stretch), and amide II (NH bend) regions are compared with the predictions of density functional theory (DFT) calculations at the M05-2X/6-31+G* level, leading to definite assignments for the H-bonding architectures of the conformers. While the C12 H-bond is present in both gamma alpha and alpha gamma, C9 rings are more prevalent, with seven of ten conformers incorporating a C9 H-bond involving in the gamma-residue. Nevertheless, comparison of the assigned structures of gas-phase gamma alpha and alpha gamma with the crystal structures for gamma alpha and larger alpha/gamma-peptides reveals that the constrained gamma-peptide backbone formed by the C9 ring is structurally similar to that formed by the larger C12 ring present in the 12-helix. These results confirm that the ACHC/ethyl constrained gamma-residue is structurally preorganized to play a significant role in promoting C12 H-bond formation in larger alpha/gamma-peptides.