SUMMARY The compositions of the aldehydes derived from phosphorus-free lipids of milk fat and ox heart, as well as from phosphatides of butter and ox heart, were investigated. In addition to normal aldehydes, considerable amounts of branched aldehydes were found, in which the branching occurred at carbon atoms situated 01, 8, and, in a few cases, y to the terminal carbon. In the literature a number of data can be found concerning the nature of the aldehydes derived from plasmalogens from various sources. Leupold (1) found octadecanal, hexadecanal, tetradecanal (probably), cis-octadecen-9-a1 and cis-octadecen-1 1-a1 released by plasmalogens from horse brain. The same aldehydes, with the exception of cis-octadecen-1 1-all were obtained by Klenk and Friedrichs (2) from plasmalogens of horse carcass and heart muscle. The presence of octadecanal, hexadecanal, and an octadecenal in aldehydes liberated from horse brain has also been demonstrated by Kaufmann and Kirschnek (3). Gray (4) subjected the aldehydes from ox heart plasmalogens, in the form of their dimethyl acetals, to gas-liquid chromatography and was able to detect 18 different components, of which two could be identified as octadecanal and hexadecanal, respectively. In a recent publication Gray (5) describes the gas-liquid chromatographic analysis of the dimethyl acetals of aldehydes derived from ox-spleen choline plasmalogens. He based a tentative identification on the retention volumes of these acetals. In addition to normal aldehydes (dodecanal up to and including octadecanal) , ten probably branched aldehydes were shown to be present. Finally, Van Duin (6) analyzed the aldehydes from butter plasmalogens in the form of their dinitrophenylhydrazones and established the presence of octadecanal and hexadecanal. In the present work the aldehydes are liberated from the phosphorus-free lipids (7) of milk fat and ox heart. After conversion of the aldehydes, via the corresponding acids, into the methyl esters, the latter are separated by gas-liquid chromatography. This separation enables tentative identification of the fractions collected. The aldehydes generated from phosphatides of butter and ox heart are similarly analyzed and compared.