The role of lignin in acetic acid production in wet oxidation (WO) of lignocellulosic biomass was discussed. It was found that lignin was indirectly attributed to the increase of acetic acid production in WO of lignocellulosic waste. A possible mechanism for this is that decomposition intermediates of lignin would act as a retardant in WO of cellulose, leading to the formation of HMF and 2-FA which are dehydration products from monosaccharides and can produce a large amount of acetic acid by their oxidation.