Soaking in aqueous ammonia (SAA) pretreatment was investigated to improve enzymatic digestibility and consequently to increase total fermentable sugar production from barley straw. Various effects of pretreatment process parameters, such as reaction temperature, reaction time, solid:liquid ratio, and ammonia concentration, were evaluated, and the optimum conditions for two of the most important factors, reaction temperature and time were determined using response surface methodology. Optimized reaction conditions were 77.6 A degrees C treatment temperature, 12.1 h. treatment time, 15 wt.% ammonia concentration, and 1:8 solid-to-liquid ratio, which gave a sugar recovery yield of 71.5 % (percent of theoretical sugar recovered from the untreated barley straw) with enzyme loading of 15 FPU/g-glucan. In the optimization of the SAA pretreatment process, ammonia concentration, reaction temperature, and reaction time were determined to be the most significant factors correlated to subsequent enzyme digestibility. Based on tested conditions exhibiting high sugar recovery yields of > 60 %, it appeared that reaction temperature affected total fermentable sugar production more significantly than reaction time.