Lone-chain E- and Z-oriented carboxylic acids, carboxylic acid esters, and fatty alcohols exhibiting a double bond in the middle part of the alkyl chain are investigated by external infrared reflection-absorption spectroscopy (IRRAS) at the air/water interface. While (Z)- and (E)-9-octadecenoic acid at from temperature form expanded monolayers with largely the same alkyl-chain order at a cation-free subphase, in the presence of Pb2+ cations, a strong condensation of the (E)-9-octadecenoic acid film is observed, implying a reduction of the amount of the gauche conformers. The order of the E-configured fatty acid is considerably enhanced, while the Z-configured acid remains strongly disordered. Possible explanations for the arrangements of the E- nd Z-configured unsaturated acid, ester, and alcohol monolayers are given both for the pure water and the Pb2+-containing subphases. In particular, evidence is presented that (E)-9-octadecenoic acid cannot achieve an "all-trans" conformation. Instead, an "anti" syn-coplanar conformation is presumably formed; i.e., one hydrogen atom of each alpha-methylene group adjacent to the double bond is positioned in the olefinic plane and the beta-carbons are oriented to opposite sides of the plane.