Rice husks, as a renewable source of energy, can provide a continuous supply of liquid and gaseous fuels through thermochemical conversion processes. However, a proper understanding of the thermal properties and reaction kinetics of rice husk plays an important role in the efficient design, operation and modeling of these systems. This paper describes the kinetics of the thermal degradation of four varieties of rice husk (Lemont LG, ROK 14, CP 4 and Pa Potho) in pure oxygen (99.5% oxygen, 50 ppm water, 40 ppm total hydrocarbons and 1 ppm solvents) atmosphere from ambient temperature to 700 degrees C at the heating rate of 20 degrees C min(-1). Two distinct reaction zones were observed for all varieties of rice husk. The kinetic parameters (activation energy, pre-exponential factor and order of reaction) were determined for the two reaction zones by applying thermo-analytical techniques to the reaction kinetics. Thermal degradation rates in the first reaction zone were significantly higher than those in the second reaction zone. The activation energies were in the range of 142.7-188.5 kJ mol(-1) and 11.0-1616 kJ mol(-1) for the first and second reaction zones, respectively.