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.