The aim of this study is to evaluate the effect of the lignin content in four hybrid poplars for enhancing ethanol production. The study was conducted using steam explosion at 200 and 220 degrees C for 5 min as a pre-treatment and then a simultaneous saccharification and fermentation (SSF) process with Saccharomyces cerevisiae. The composition of raw material, liquid, and solid fraction obtained after pretreatment, enzymatic digestion, and ethanol production under the different experimental conditions was analyzed. The best results for bioethanol production were obtained from steam explosion pre-treatment carried out at 220 degrees C with the hybrid poplar H-29, with cellulose recovery of over 63%, enzymatic hydrolysis yield of approximately 67%, and SSF yield of 70% of the theoretical value. However, the highest enzymatic hydrolysis yield (79%) was obtained for the hybrid poplar H-34, which has the lowest lignin content.