Eighteen crude oils from Kuwait oil fields were analyzed by capillary GC/MS spectroscopy. Thirty-two biomarker compounds, 18 pentacyclic triterpanes and 14 steranes were identified and semi-quantitatively determined by selective ion monitoring (SIM) chromatography, The data obtained were used to calculate several biomarker indices. Depending on the oil source, the variation in the pentacyclic triterpanes distribution is more significant than the variation in the sterane distribution. The C-29 norhopane and C-30 hopanes were the dominating biomarkers in all the crude oils analyzed. The calculated biomarker indices indicated that the analyzed oils stem from medium mature oil reservoirs that have undergone biodegradation, to a certain extent.