C-1-C-30 products from Fischer-Tropsch synthesis, conducted in a supercritical n-hexane medium over an Fe catalyst in a fixed-bed reactor, are analyzed using on-line gas chromatography. A Hewlett-Packard 5890 Series II gas chromatograph (GC) is modified to minimize the effects of condensation of the on-line sample in the transfer lines. The GC is configured with a Supelco Petrocol DH capillary column connected to a flame ionization detector (FID) and two 1.83 m x 3.18 mm stainless steel columns placed in series, packed with. 80/100 mesh HayeSep D, connected to a thermal conductivity detector(TCD). It is shown that pressure and temperature affect the elution order of oxygenates relative to hydrocarbons in the nonpolar capillary column. This phenomenon is exploited for obtaining improved resolution; several distinct methods produce similar elution orders. Ar, added to the syngas feed, is used to calculate syngas conversion. All compounds eluting before hexane (C-1-C-5, Other than 2-methylpropene/1-butene and propanal/propanone) and nearly all the major peaks eluting after hexane are resolved in the capillary column. H-2, Ar, CO, CH4, CO2, and H2O are resolved in the packed columns. The method provides excellent quantitative measurement of component mole fractions that are within the range of calibration.