The degradation of xylan during methane fermentation proceeded as a first-order reaction, The rate constants were calculated to be 0.40-0.09 day(-1) at 37 degrees C and 0.341 day(-1) at 55 degrees C. From calculations based on the experimental data, K-A and C-A values in the expression of the velocity of xylose consumption changed as the fermentation progressed. In the mesophilic fermentation, the degradation of xylan slowed down after 2 days of incubation, but the rate of consumption of xylose increased between days 3 and 4 of incubation and slow again at the 5th day of incubation, In the thermophilic fermentation, the degradation of xylan proceeded at a constant rate and the rate of consumption of xylose increased slightly on the 3rd day of incubation. When the velocity of gas evolution was determined, the C, value for acetate at 55 degrees C was about 1.8 times larger than the value at 37 degrees C.