Low cost fermentation substrates frequently contain a mixture of carbon sources including hexoses, pentoses and disaccharides. Fermentation of such mixtures requires an understanding of how each of these substrates is utilized. During batch culture of Pichia stipitis CBS 5776 on sugar mixtures, glucose causes catabolite repression of xylose and cellobiose utilization. Also, glucose causes a permanent repression of xylose utilization as evidenced by reduced growth rates during the xylose phase of glucose/xylose fermentation. The growth model for multiple substrates is developed based on a cyclic AMP mediated catabolite repression mechanism and this model adequately described the growth and ethanol production from sugar mixtures.