Gas Chromatographic retention data for the methyl esters of acyclic isoprenoid acids are discussed and shown to be useful in their recognition. Analytical methods included the use of a combined gas chromatograph-mass spectrometer; fragmentation patterns for the methyl esters of acyclic isoprenoid acids are briefly noted. Two shales of different age and different depositional history have been examined for their isoprenoid acid content. Serpiano Oil Shale, (Triassic, ca 210 × 10 6 yr; marine) from Switzerland contains all of the isoprenoid acids from C 11-C 20 (and possibly C 21), the n-adds from C 8-C 23 and a series of ’pseudo‘ isoprenoid acids. Green River Shale (Eocene, ca 60 × 10 6 yr; lacustrine) from Colorado, U.S.A. contains the C 14-C 17, and C 19-C 22 isoprenoid acids and the n-acids from 012–034. The range of α,ω-dicarboxylic acids is considerably extended over previous findings and a series of 2-alkylsuccinic acids is provisionally reported.