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This paper reports on a new method for rapid determination of organic matter (OM) in soil and municipal sludge samples, based on a redox reaction (i.e., conversion of OM into CO2 by dichromate), involving the headspace gas chromatographic technique (HS-GC). For the soil samples (with a lower OM content), the sample reaction was conducted in a sealed headspace vial under the given conditions (135 °C for 20 min) and the CO2 from the reaction is directly measured by HS-GC. For the municipal sludge samples (i.e., with a higher OM content), the sample reaction was conducted in an open vial under the above conditions. Then, a portion of the reaction medium was taken and added into a sealed vial containing an excess amount of oxalic acid, followed by HS-GC measurement, from which the residual amount of potassium dichromate in the reaction medium can be determined and thus the OM content in the sample can be calculated. The results showed that the present method has good reproducibility and accuracy, with a relative standard deviation < 5% and recovery between 91.8% and 106%. Based on a headspace auto-sampler, high-throughput determination of the OM content for batch sample analysis can be realized.