Raman spectra of dihexadecylphosphatidic acid (DHPA) and of dimyristoylphosphatidylcholine (DMPC) and its longer chain homologues have been obtained as a function of temperature in order to study the conformational order of the hydrocarbon chains in lipid bilayers. The frequency of the longitudinal acoustical (LA) vibration band is evaluated in terms of the length of ail-trans chain segments. In the ordered phase, the chains are found to be overwhelmingly in the all-trans conformation. In the fluid phase, definite all-trans segments occur predominantly, the length of which coincides with the extension of the order parameter plateau known from deuterium magnetic resonance (DMR). The frequency of the skeletal optical (SO) trans vibration band leads to the same result, if evaluated under the assumption of vibrational decoupling by gauche bands in the fluid phase, thus lending support to this assumption. The intensity of this band determined from the band area increases linearly with chain length in the ordered phase and is independent of chain length in the fluid phase. Evaluating the intensity for the length of all-trans segments, the same result for the chain conformation is obtained as derived from the frequencies, with the additional information that the length of the all-trans segments in the fluid phase does not vary with chain length.