The aim of this work was to study the ability of Saccharomyces cerevisiae (baker's yeast) to metabolize a variety of aromatic compounds found in rice straw (RSHH) and olive tree pruning (OTHH) hemicellulosic hydrolysates, obtained by acid hydrolysis at different sugar and toxic compound concentrations. Initially, the hydrolysates were inoculated with S. cerevisiae (10 g L(-1)) and incubated at 30 °C under agitation at 200 rpm for 6 h. The results showed that this yeast was able to utilize phenolic and furan compounds in both hemicellulose hydrolysates. Next, the treated hydrolysates were inoculated with Pichia stipitis NRRL Y-7124 to evaluate the effect of biotransformation of aromatic compounds on ethanol production, and better fermentation results were obtained in this case compared to untreated ones. The untreated hemicellulose hydrolysates were not able to be fermented when they were incubated with Pichia stipitis. However, in RSHH treated hydrolysates, ethanol (Y(P/S)) and biomass (Y(X/S)) yields and volumetric ethanol productivity (Q(P)) were 0.17 g g(-1), 0.15 g g(-1) and 0.09 g L(-1) h(-1), respectively. The OTHH-treated hydrolysates showed less favorable results compared to RSHH, but the fermentation process was favored with regard to untreated hydrolysate. These results showed that the fermentation by P. stipitis in untreated hydrolysates was strongly inhibited by toxic compounds present in the media and that treatment with S. cerevisiae promoted a significant reduction in their toxicities.