Vibrational sum-frequency spectroscopy has been employed to examine the vibrational spectra, and determine the conformational order of diacylphosphatidyl lipid monolayers at a water-oil interface. These nonlinear vibrational spectra provide direct information about the orientation and the degree of order of the acyl chains of the lipid monolayers. Halothane was introduced into the lipid monolayers, and the resulting changes in acyl chain order were examined. Both neutral (zwitterionic) and charged lipid monolayers were studied in an effort to determine the likely sites of interaction between halothane and the lipid molecules. Two chain lengths (14 and 16 carbons) and four headgroups (choline, ethanolamine, glycerol, and serine) were studied. Each monolayer exhibits significant disorder, and the conformational order of each monolayer is perturbed after the introduction of halothane.