The theoretical stereoconfiguration of the possible isomers has been indicated by Nichols et aZ.3. Swift et al.have isolated from cottonseed oil (which contains approx. 45 per cent linoleic acid) in various stages of autoxidation 2,4-decadienal, 2-octenal, and hexanal. Recently, Patton et aL&have published results on the occurrence of 2,4-decadienal in several oils and fats. The formation of these decomposition products can be readily explained by means of the above-mentioned theory. From the relevant literature, however, no information can be obtained on the stereo-configuration of the unsatur-ated aldehydes obtained, notably 2,4-decadienal. Fig. 1. Gas-liquid chromatography of: a, palm oil decomposition product (detail); b, groundnut oil, ditto; c, synthetic 2(trans)~ 4(ci)-decadienal; d, synthetic 2(rans)-4(ran f)-decadienal. Carrier, 'Celite 545'; immobile phase, silicone oil (30 per cent); mobile phase, nitrogen; pressure, 50 cm. mercury; gas velocity, 50 ml./min.; temperature, 137 C.; length of column, 90 cm.; detector, gas density balance From our investigations with palm oil and groundnut oil it appeared that on subjecting the volatile decomposition products to gas-liquid chromatography6, two aldehydes could be isolated, which were identified as two isomeric 2,4-decadienals (Figs. la, b; 2a, b). By partition chromatography of the dinitrophenylhydrazones, following the method of Miss Kramer and van Duin7, the 2,4-decadienal structure could be established, but not the stereo-configuration.Two isomeric 2,4-decadienals of known stereo-configuration were synthesized. Using Brown and Fig. 2. Infra-red spectra of: a, 2(ra/is)-4(ci )-decadienal from palm oil (first peak in Fig. la); b, 2(irws)-4(ran )-decadienal from palm oil (second peak in Fig. la); c, synthetic 2(trans)-4: (cis)-decadienal (cf. Fig. Ic); d, synthetic 2(iram)-4(nms)-de- cadienal (cf. Fig. Id).To record these spectra, a 'Unicam' 100, double-beam infra-red spectrophotometer was employed (rock-salt prism, microplatetechnique) Subba R-ao's method8 2(rans)-4(c s)-decadienal was prepared from 2(rans)-4(cis)-decadienoic acid. This acid was isolated, in a yield of 3-5 per cent, from Stillingia oil, the oil from the seeds of Sapium sebi-ferum Roxb. of which the stereo-configuration has been established by Crombie9. By application of Doebner's method (cf. Forss and Hancox10) 2(trans)-4(rans)-decadienal was synthesized, starting from hexanal. As the synthetic products and the two aldehydes isolated from volatile decomposition products gave similar results in the gas-liquid chromato-graphic analysis (Figs. Ic, d) and in the infra-red-analysis (Figs. 2c, d), the conclusion is justified that the two natural 2,4-decadienals obtained possess the 2(trans)-4:(cis) and 2(trans)-4:(trans) stereo-configuration. From the gas chromatogram these transis and transjrans compounds were found to be present in the ratio of 28: 72. The above affords new, indirect proof that the (la) isomer of linoleic acid hydroperoxide is formed as one of the primary autoxidation products as found spectrophotometrically by Sephton and Sutton11. Then it must be assumed that isomer (la) serves as precursor for 2(rans)-4(cts)-decadienal. The more stable 2(rans)-4(rans)-deeadienal can be formed, either as a secondary reaction product during further rearrangement of isomer (la) to 9-hydroperoxy-10(2rans)-12(rans)-decadienoic acid, or as a result of the instability of 2(rans)-4(c s)-deeadienal. More detailed results will be published elsewhere. We wish to thank Mr. J. H. Recourt for his assistance in carrying out the gas-liquid chromatography experiments. Further, our thanks are due to Unilever Research Department, Port Sunlight, for supplying the sample of Stillingia fatty acids and to the management of Unilever N.V., Rotterdam, for permission to publish these data.