In this work, rapeseed straw was pretreated with FeCl3 to achieve high sugar recoveries. Temperature (120-160 degrees C), and FeCl3 concentration (0.1-0.3 M) were selected as factors and modified according to a central composite experimental design. The pretreatment conditions were expressed using the combined severity, which ranged from -0.12 to 2.29. Considering a double criterion that maximizes simultaneously the recovery of hemicellulosic sugars in the liquid fraction from pretreatment and the enzymatic hydrolysis yield, the optimal conditions were found to be 138 degrees C and 0.25 M salt concentration. The FeCl3 pretreatment of rapeseed straw under these optimized conditions resulted in 75% hemicellulosic sugar recovery and 53% enzymatic hydrolysis yield. Thereby, 100 g dry rapeseed straw yielded 37.8 g sugars, equivalent to 70% maximum potential sugar in rapeseed straw.