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Major conversion pathways involved in AHP pretreatment of softwood and glucomannan were elucidated. An endwise depolymerization reaction, accompanied with termination reactions, is the predominant carbohydrate degradation pathway. Considerable amounts of carboxylic acids including lactic acid, glycolic acid, succinic acid and formic acid were produced along with pretreated biomass. AHP pretreatment of softwood can remove a significant amount of lignin and glucomannan (22% and 78% wt, respectively) while cause little degradation on cellulose. This study demonstrated that AHP pretreatment may provide a potential process to convert hemicellulose to valuable organic acids along with the pretreatment of biomass for cellulosic biofuels production.