Enzymatic hydrolysis of hybrid poplar treated by ammonia recycle percolation (ARP) was studied applying cellulase enzyme supplemented with additional xylanase or pectinase. The effect of xylanase addition was much more significant than pectinase addition. Conversion of ARP-treated hybrid poplar to ethanol was carried out by simultaneous saccharification and fermentation (SSF) and SS and cofermentation (SSCF). The maximum ethanol yield observed from the SSCF experiment was 78% of theoretical maximum based on the total carbohydrate (glucan + xylan). The same feedstock was also treated by soaking in aqueous ammonia (SAA), a batch pretreatment process with lower severity than ARP. The test results indicated that relatively high severity is required to attain acceptable level of digestibility of hybrid poplar. In order to lower the severity of the pretreatment, addition Of H2O2 was attempted in the SAA. Addition of H2O2 significantly enhanced delignification of hybrid poplar due to its oxidative degradation of lignin. Several different H2O2 feeding schemes and different temperature profiles were attempted in operation of the SAA to investigate the effects of H2O2 on degradation of lignin and carbohydrates in hybrid poplar. More than 60% of lignin in hybrid poplar was removed with stepwise-increase of temperature (60-120 degrees C after 4h of reaction). Increase of carbohydrate degradation was also observed under this condition. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 357-364, 2009