Abstract: Series of di-and triunsaturated ketones in marine surface sediments from South China Sea and Okinawa Trench are identified in varying amounts by thinlayer chromatography, gas chromatography and computerised gas chromatography-mass spectrometry.They are C_{37 3}Me, C_{37 2}Me.C_{38 3}Et, C_{38 3}Me, C_{38 2}Et, C_{38 2}Me. C_{39 3}Et and C_{39 2}Et respectively. Electron impact high resolution mass spectrometry of the appropriate TLC-fractions(Rf= 0.45-0.80) revealed that the alkenones have molecular ion peaks at m/e 528. 530, 542. 544. 556. and 558 and abundant peaks at m/z 55. 57.67. 71. 82. 96.etc in their mass spectra. The alkenone of C_{37 2}Me is the most abundant component in the alkenones. The fact that the undetection of alkenones in the modern sediments from Antarctic suggests that the abundances of long- chain alkenones in sediments are related to the geographical distribution of some marin organisms such as ? huxleyi. The distribution of the alkenone relative abundances of the four samples is very similar which implies that alkenones appear to be restricted to a few species of the class Prymnesiophyceae. notably cocclithophorids of the family gephyrocapsaceae. The degree of unsaturation of alkenones decreases as growth temperature increases. The relation between unsaturation index U₃₇^k(=C_{37 2}/[C_{37 2}+C_{37 3}]) calculated on the amounts of C_{37 2}Me and sea surface temperature is as follows: U₃₇^K=0.034T+0.039 Sea surface temperatures estimated on Up are close to the values measured in the field. This suggests that long- chain alkenones cas be used as a geochemical tool to estimate not only past surface water temperatures but also environmental conditions of sediment deposition.