Organic solvents have been shown to affect the activity and selectivity of enzymes and whole cells in biphasic and nearly anhydrous systems. The choice of supercritical solvents and operating conditions may be used to vary the rates and selectivities of enzymes. Experiments were conducted to assess the impact of compressed nitrogen, ethane, and propane on the product selectivity of whole cell biocatalysts. The anaerobic thermophilic bacterium Clostridium thermocellum, which produces ethanol, acetate, and lactate, was used in the biphasic incubations. Compressed solvents lead to an increase in the ratio of ethanol to acetate produced by the organism. Furthermore, lactate formation was decreased in the presence of compressed and liquid solvents. The reduction in lactate formation is associated with a simultaneous reduction in the rate of cellobiose uptake by the microorganism. These results support the claim that incompatible solvents mag affect the membrane and membrane transport systems of the microorganism. Finally, this work shows the potential for the manipulation of the product selectivity of whole cell biocatalysts by tuning solvent properties.