Decomposition rates of peat soil in temperate mires were measured by peat mass loss, cellulolysis, CO₂ emission and O₂ consumption rates with reference to vegetation. Measurements were made in the Masukata mire (mire in pond) and Sasakami mires (mires at knick points) in central Japan. Peat mass loss and cellulolysis rates were measured by using litter bags with 0.25 mm mesh. O₂ consumption rate was measured by the O₂ exchange within a closed chamber placed on peat surface. CO₂ emission rate of peat was measured under incubation at 25 °C. Peat mass loss rate did not show significant difference between vegetation types. Correlation between peat mass loss and CO₂ emission rates was significant. Cellulolysis rate was significantly different between vegetation types. The rate at Alnus and Phragmites communities were significantly higher than Sphagnum community, whereas the rate in the Rhynchospora community was significantly lower than the Sphagnum community. Correlation between cellulolysis rate and O₂ absorption rate was significant. Cellulolysis rate could be the measure of the potential decomposition rate of organic materials by the aerobic decomposition process. Peat mass loss rate could be the measure of the actual decomposition rate including anaerobic processes and the rate depends on the microbial community specific to peat quality or peat forming materials.