A series of hydrophobic peptides K-2(LA)(x) (x = 6, 8, 10, 12) has been synthesized. IR and CD studies in MeOH solution are reported, along with IR studies of these species in vesicles with 1,2 dipalmitoylphosphatidylcholine, and IR Reflection-Absorption Spectroscopy (IRRAS) studies of peptide and lipid/peptide monolayer Films in situ at the air/water interface, In bulk phases, tile propensity toward helix formation increases with increasing chain length, there being essentially no helix in the shortest peptide, varying and concentration-dependent helical content in K-2(LA)(8), and >90% helix formation in both K-2(LA)(10) and K-2(LA)(12). In monolayers at the air/water interface, peptide secondary structure was inferred from both the Amide I and Amide A bands. The shortest peptide adopted an antiparallel beta-sheet structure, while the remainder of the series (when spread at low surface pressure) appeared to adopt varying proportions of parallel beta-sheet forms, The secondary structure adopted by K-2(LA)(10) and K-2(LA)(12) depended remarkably on the initial spreading pressure; when spread at high pressures, the molecules were alpha-helical. The current experiments demonstrate the unique advantages of IRRAS for evaluation of peptide conformations in situ at the air/water interface and reveal large differences in the propensity for helix formation in monolayers compared with bulk phases.