The low ethanol tolerance of thermophilic anaerobic bacteria (< 2%, v/v) is a major obstacle for their industrial exploitation for ethanol production. The ethanol tolerance of the thermophilic anaerobic ethanol-producing strain Thermoanaerobacter A10 was studied during batch tests of xylose fermentation at a temperature range of 50-70 degrees C with exogenously added ethanol up to similar to 6.4% (v/v). At the optimum growth temperature of 70 degrees C, the strain was able to tolerate 4.7% (v/v) ethanol, and growth 1 was completely inhibited at 5.6% (v/v). A higher ethanol tolerance was found at lower temperatures. At 60 degrees C, the strain was able to tolerate at feast 5.1% (v/v) ethanol. A 1 generalized form of Monod kinetic equation proposed by Levenspiel was used to describe the ethanol (product) inhibition. The model predicted quite well the experimental data for the temperature interval 50-70 degrees C, and the maximum specific growth rate and the toxic power (n), which describes the order of ethanol inhibition at each temperature, were estimated. The toxic power (n) was 1.33 at 70 degrees C, and corresponding critical inhibitory product concentration (P-crit) above which no microbial growth occurs was determined to be 5.4% (v/v). An analysis of toxic power (n) and P-crit showed that the optimum temperature for combined microbial growth and ethanol tolerance was 60 degrees C. At this temperature, the toxic power (n), and. Pc,it were 0.50, an 6.5% (v/v) ethanol, respectively. From a practical point of view, the model may be applied to compare the ethanol inhibition (ethanol tolerance) on microbial growth of different thermophilic anaerobic bacterial strains.