A critical factor in simultaneous saccharification and fermentation (SSF) is the selection of process temperature, which needs to be a compromise between the optimal temperature for enzymatic hydrolysis and that for fermentation. In the present work, isothermal and nonisothermal SSF was experimentally studied using pretreated spruce and arundo materials at a loading of 10% water insoluble solids, with an industrial strain of Saccharomyces cerevisiae as the fermenting organism. In the nonisothermal experiments, the temperatures were linearly increased during the batch time of 96 h, and the results were compared to isothermal SSF operation. The final ethanol concentrations obtained for the arundo material was 16.5 g/L in the nonisothermal process using an initial temperature of 32 degrees C and a linearly increasing temperature of 0.135 degrees C/h. As a comparison, the final ethanol concentration obtained was 13.4 g/L for an isothermal operation at 32 degrees C and 15.3 g/L for an isothermal process running at 39 degrees C. The corresponding values for the spruce material were 33.1 g/L, and 29.1 g/L and 32.2 g/L, for nonisothermal and isothermal operation at 32 and 39 degrees C, respectively. The obtained ethanol yields, in particular for the case of arundo, demonstrate that nonisothermal SSF operation can give increased ethanol yields in comparison to isothermal SSF.