The maximum concentration of water-insoluble solids (WIS) in simultaneous saccharification and fermentation (SSF) is restricted due to inhibition of the enzymes and the yeast, as well as mass transport problems caused by the viscosity of the pretreated material. However, the higher the concentration of WIS during SSF the less energy is needed in the subsequent distillation and evaporation steps. In this study, SSF was performed on barley straw sprayed with H2SO4 and steam pretreated at conditions yielding a highly digestible material, aiming to increase the WIS concentration and decrease the enzyme loading and the yeast concentration in SSF, in order to reduce the production cost. Three concentrations of WIS (5, 7.5 and 10%), and three enzyme loadings (5, 10 and 20 FPU/g cellulose) of Celluclast 1.5 L complemented with Novozym 188 were investigated in terms of ethanol yield. Ordinary cultivated Baker's yeast and Baker's yeast cultivated on barley straw hydrolyzate were also evaluated in terms of ethanol yield. The highest ethanol yield, 82% of the theoretical based on the glucose content in barley straw, was obtained after SSF with 5% WIS at an enzyme loading of 20 FPU/g cellulose together with 5 g/L ordinary cultivated yeast. Increased WIS concentration and decreased enzyme loading decreased the ethanol yield. However, by cultivating the yeast in hydrolyzate from pretreated barley straw the WIS concentration in SSF could be increased from 5% to 7.5% and the yeast concentration could be reduced from 5 to 2 g/L, while still attaining a yield of approximately 80%. (c) 2006 Elsevier Inc. All rights reserved.