The furan aldehydes furfural and HMF, which are formed by dehydration of pentose and hexose sugars, respectively, are commonly found in lignocellulose hydrolysates (Jönsson et al., 2013). In present study, HMF and furfural decreased with increased magnesium bisulfite dose. An explanation is that more magnesium bisulfate depressed the decomposition of the sugar to HMF and furfural in a relatively higher pH (5.2–2.7). A similar phenomenon was also discovered in SPORL pretreatment (Zhu et al., 2009). The data in Table 1 also indicated that increasing pretreatment temperature and pretreatment time led to the production of HMF and furfural as expected. At high temperature, HMF would be further degraded and form levulinic acid and formic acid, the latter was also formed from furfural under acidic conditions (Palmqvist and Hahn-Hägerdal, 2000). Only little formic acid and levulinic acid were detected in the pretreatment spent liquor while magnesium bisulfite dose was in a low level (1.76%).