A vibrational and attendant conformational analysis from Raman and infrared spectra measured respectively in the range 100-3200 cm-1 and 400-3700 cm-1 of molten oleic and elaidic acids, i.e. Z and E configurations of DELTA-9 octadecenoic acid, and of n-nonanoic acid is presented. A comparative study of the most sensitive vibrations to alkyl chains conformational changes : C-C stretching modes (950-1150 cm-1), deltaCCC deformation (particularly D-LAM) 150-350 cm-1) and rocking methyl deformation, r(parallel-to)CH3, (800-980 cm-1) lead us to the following results. In the three pure liquid compounds, the dimeric carboxylic group conserves a C(i) local symmetry and couples, as in the solid state, the vibrational modes of the two nine carbon polymethylenic chains located at both sides; the degree of coupling depends on the type of vibration. For the skeleton stretching vibrations the overall C18 central pseudo-paraffinic segment is involved; the frequencies measured, close to those of the solid state lend support to a central C18 nearly fully extended rotamer. On the contrary, for the longitudinal acoustic mode (LAM1), more sensitive to the C18 planarity distorsions, a D-LAM (D for disorder) appears : its frequency shows that the most frequent nonlocalizable all-trans segment consists of 10-11 carbon atoms with a C18 pseudo-paraffinic average statistical disorder decreasing from nonanoic acid to the oleic and elaidic acids. Consequently, the 1090 cm-1 Raman and infrared signal, characteristic of a chain with "left-handed" defects, only intense in the olefinic acids spectra, appears to be due to their C9 strongly disordered methylated segments. The r(parallel-to)CH3 attributions agree with this result : in the olefinic acids, the end chain defects GT(n-4), TGT(n-5), G+/-TG+/-T(n-6) are observed whereas in the nonanoic acid the only one significatively present is the GT(n-4) defect. Finally, in the amorphous state, oleic and elaidic acids are constituted by a relatively ordered upper half, i.e. the half nearest the head group, and a more disordered lower half; this conformational behaviour partly explains why in a suited aqueous environment, amphiphilic oleic or elaidic acid molecules join together in ordered packing with a sufficient degree of fluidity, to form bilayer structures as myelin tubes, i.e. simple models of biological membranes.