Vibrational sum frequency spectroscopy in conjunction with interfacial pressure measurements provides direct information about the molecular structure of phosphocholine monolayers adsorbed to the interface between D2O and carbon tetrachloride. Monolayers form from breakup at the interface of aqueous phase phosphocholine vesicles. For the saturated, symmetric, dialkylphosphocholines used in this study, alkyl chain conformation as inferred from the relative intensity of CH stretch vibrational bands depends on both alkyl chain length and interfacial concentration. Temperature-controlled experiments show the lipid bilayer gel to liquid crystalline phase transition temperature to play a pivotal role in determining interfacial coverage and alkyl chain structure. At equivalent interfacial coverages, longer chain phosphocholine species form more disordered monolayers than shorter chain phosphocholines.