AbstractThe exact position of the double bond and the probable branch assignments of each hydrocarbon isomer ofSarcina lutea andSarcina flava were determined by derivatives and gas chromatography and combined gas chromatography‐mass spectrometry. It was shown by mass spectral data that in all isomers the double bond position was in or near the center of the molecule and, that some of the resolved isomers contained an additional positional isomer. The hydrocarbons were identified as containing methyl branches in the iso or anteiso or both configurations, symmetrically and asymmetrically disposed on the ends of the isomers. The assignment of the methyl branches for each tetrad of four isomers as the isomers emerged from the gas chromatographic column was iso‐iso’; anteiso‐iso; anteiso‐anteiso’; and isonormal for odd carbon‐numbered chains, and iso‐iso’; anteiso‐iso; iso‐normal; and anteiso‐normal for even carbon‐numbered chains. A fifth isomer was identified as a normal‐olefin.