Photocatalyst-assisted ammonia pretreatment was explored to improve lignin removal of the lignocellulosic biomass for effective sugar conversion. Corn stover was treated with 5.0-12.5 wt.% ammonium hydroxide, two different photocatalysts (TiO2 and ZnO) in the presence of molecular oxygen in a batch reactor at 60 A degrees C. Various solid-to-liquid ratios (1:20-1:50) were also tested. Ammonia pretreatment assisted by TiO2-catalyzed photo-degradation removed 70 % of Klason lignin under the optimum condition (12.5 % ammonium hydroxide, 60 A degrees C, 24 h, solid/liquid = 1:20, photocatalyst/biomass = 1:10 with oxygen atmosphere). The enzymatic digestibilities of pretreated corn stover were 85 % for glucan and 75 % for xylan with NH3-TiO2-treated solid and 82 % for glucan and 77 % for xylan with NH3-ZnO-treated solid with 15 filter paper units/g-glucan of cellulase and 30 cellobiase units/g-glucan of beta-glucosidase, a 2-13 % improvement over ammonia pretreatment alone.