Inhibitory effects of ethanol and glucose on a high-ethanol-tolerance yeast strain (fusion product of Saccharomyces diastaticus and Saccharomyces uvarum) having high osmotic and ethanol tolerance were studied in batch cultures. A model incorporating both substrate and product inhibition was developed that represented the experimental data quite well. By performing fed-batch fermentation, an ethanol concentration of 13.3% (w/v) was obtained. The maximum allowable ethanol concentration for cell growth was predicted to be 129.9 g/L and ethanol-producing capability of cells was found to be completely inhibited at 136.4 g/L. On-line monitoring of the fermentation was performed using an ion trap mass spectrometer and a triple quadrupole mass spectrometer. Preliminary results are reported.