Glucose and xylose were used as mixed carbon sources for ethanol production by a cascade immobilized-cell reactor. In the first fixed-bed reactor, immobilized S. cerevisiae was used to metabolize the glucose anaerobically, and in the second fixed-bed column, the immobilized Pichia stipitis was applied to metabolize xylose aerobically. The effect of xylose on the glucose uptake by S. cerevisiae and the effect of ethanol on the xylose metabolization by P. stipitis were studied. A mathematical model taking both the substrate and xylose inhibition into consideration was suggested for the description of the two-substrate fermentation process, and a total effectiveness factor eta(s) in the model was derived to take into account the mass transfer resistances of intra-particles and liquid films. Calculated results are in fair agreement with the experimental data.