In situ decomposition of Malaysian peat soil organic matter in the field was estimated by the measurement of the CO2 flux from the soil surface. The CO2 flux ranged from 5.8 to 30.3 mmol h-1m-2, equivalent to 0.7 to 3.6 kg carbon per hour per ha. The CO2 flux was correlated with the soil acidity and ash content: the higher the soil pH and/or ash content, the greater the CO2 flux. Annual surface subsidence of arable peatland caused by the microbial decomposition of peat soil was estimated by the CO2 flux measurement to be 50-70% of the whole annual surface subsidence. Annual amount of CO2 emission from arable peatland in Peninsular Malaysia was estimated to be 2.23 × 106 t of carbon.