A thermostable alkaline peroxidase was partially purified from rice hulls by precipitation in 70% (v/v) isopropanol, anion exchange chromatography on a DEAE cellulose column (eluted by 50 mM potassium phosphate, pH 6.0), and gel filtration on a Sephacryl S-200 column. The peroxidase (RHP) showed a maximum activity at a slightly alkaline condition, between pH 7 and 8, for the oxidation of guaiacol in the presence of 0.2 mM H2O2. The half life time for the inactivation of RHP at 68 degrees C was 168 min nearly six times that of horseradish peroxidase (HRP) at the same temperature. Dioxane enhanced the activity of RHP but decreased that of HRP.