Thermoanaerobacter thermohydrosulfuricus Rt8.B1 exhibited hyperbolic growth (i.e. a continuous rate of growth, without diauxie, during growth and utilization of two carbon sources) on mixed carbohydrate substrates when grown in pH-controlled batch culture. Hyperbolic growth was observed with xylose in combination with either glucose or cellobiose. Diauxic growth ways observed when T. thermohydrosulfuricus Rt8.B1 was grown on a glucose plus cellobiose substrate mix. The major fermentation end-products under all substrate conditions were ethanol and acetate. Ethanol production varied depending on the substrate supplied and was always greatest on mixtures that included xylose (i.e. hyperbolic growth). High ethanol-to-acetate ratios could not be explained on the basis of a greater substrate uptake and thus more ethanol production under these conditions, or by variations in the levels of acetate kinase and NADP-linked alcohol dehydrogenase synthesis. The high ethanol-to-acetate ratio could not be increased by growing T. thermohydrosulfuricus Rt8.B1 under a partial pressure of hydrogen (1 atm) or by growth at different pH. Growth under these conditions decreased the ethanol-to-acetate ratio.