The growth of Debaryomyces nepalensis on mixed substrates in batch culture follows diauxic pattern of completely utilizing glucose during the first exponential growth phase, followed by an intermediate lag phase and a second exponential growth phase consuming xylose. The present study deals with the development of a cybernetic mathematical model for prediction of xylitol production in the presence of co-substrate. To experimentally validate the model, defined media supplemented with different concentration of co-substrate were tested and compared with production on single carbon source. High xylitol yield of 0.73 g g(-1) was achieved when supplemented with 10 g L(-1)of glucose in the xylose-rich media. The proposed model could adequately describe the diauxic growth, sequential utilization of substrates and production of xylitol under various ratio of co-substrate. The model could be used as a tool to assess the influence of co-substrate (glucose) concentration added to the media on xylitol production.