The oil lakes in southern Kuwait have accumulated oil that was spilled during the 1991 hostilities and have been exposed to processes of surface degradation for the last decade. Long-chain normal alkanes are abundant in these oils, showing that biodegradation has not been a major factor, whereas the absence of most compounds with less than 10 carbon atoms suggests a significant role for evaporation. Evaporation has been simulated in laboratory experiments at 25, 30, 40, and 50°C and compositional changes monitored by taking samples periodically for gas chromatographic analysis. Evaporation is initially rapid but slows through time, and ultimately, most compounds with less than 10 carbon atoms are lost. For a given carbon number, loss proceeds in the sequence: normal alkanes> branched chains> naphthenes> aromatics (which is the opposite of water washing). GCMS data for hopane and sterane biomarkers confirm that oil in the lakes is derived from the giant Burgan oilfield and also show that these biomarkers are not affected by evaporation in either nature or the laboratory. Analysis of sulfur compounds using GC-FPD showed that oils exposed in the lakes were photo-oxidized and had reduced concentrations of benzothiophenes and increased volatile sulfur compounds. A loss of volatile hydrocarbons from the free surface leads to compositional layering unless the oil is well mixed by convection or diffusion. In experiments to monitor the development of layering, low molecular weight compounds were rapidly lost from the surface, and a steep compositional gradient developed. The formation of a devolatilized, viscous surface “skin” tends to make evaporation a self-limiting process and also has significance for the design of sampling protocols in environmental forensics.