An IR study of the conformational disorder in the part of the hydrocarbon chain nearest the ester group in gel-phase lipid bilayers is presented. The infrared method for determining the conformation of the CH2-CH2 bond attached to the ester group is discussed and applied to a number of representative gel-phase 1,2-diacylphosphatidylcholine fully hydrated dispersions, including symmetric and mixed-chain PCs in their gel and subgel phases. The effect on chain conformation of the headgroups PC, PE, and PA(-) was explored. Only two conformations of the alkyl chains were found: a planar (all-trans) chain and a nonplanar chain that is all-trans except for the CH2-CH2 bond nearest the ester group. Concentrations of the two conformers were also determined. The overall concentration of the planar form always exceeds 50% but varies significantly depending on the lipid involved. In two cases, the concentration of the planar conformer was estimated for both the SN1 and SN2 chains.