This paper reports on a robust method to quantify the acetyl group in biomass and its derived liquid or solid samples using headspace gas chromatography (HS-GC). The method was based on the sample pretreatment in an oxalic acid medium (0.6 mol/L) at 150 degrees C and 10 min for the liquid samples or 70 min for the solid samples to convert the acetyl group to acetic acid, which can be quantified by a full evaporation HS-GC technique. It was found that similar to 100 mu m was the suitable solid particle size to be used in the sample analysis at the suggested pretreatment condition. The results showed that the present method has good measurement precision (relative standard deviation was less than 2.09% for the liquid sample and 2.83% for the solid sample) and accuracy (with the recoveries of 96.2-104% in the acetyl group quantification). The limit of quantification of the method was 163 mg/L for the biomass liquid samples and 0.11% for the biomass solid samples. The present method can be a valuable tool to provide the high-throughput testing for the acetyl group in biomass and its derived samples in biorefinery-related development.