A new method for equilibrating water containing alkane hydrocarbons with a gas phase and analyzing the gas for hydrocarbon content by gas chromatography extends analytical sensitivity to better than 0.1 part per billion. The solubilities at 25 degrees C of the normal C(9) (220 parts per billion) and C(10) (52 parts per billion) alkanes decrease with increasing carbon number. A discontinuity occurs at the normal C(11), alkane, probably because of a change from true solubility (molecular dispersion) to accommodation (aggregation).