Three NaOH doses of 3.0%, 5.0% and 7.0% based on dry matter of poplar processing residues were used in this study. The moisture contents (MC) was 88%, similar to that in the report of Zheng et al. (2009). No NaOH addition treatment was used as control. All the prepared samples were kept at ambient temperature (20 ± 1 °C) for 4 days. A few pretreated samples were dried in an electronic oven at 60 °C for 48 h and then kept in a refrigerator for chemical composition analyses (Pang et al., 2008).