The feasibility of improving the biodegradability of refractory pollutants in sub- and supercritical water was investigated. Poly vinyl alcohol (PVA) was chosen as a test material to represent high-molecular-weight refractory pollutants. At a temperature of 350degreesC, BOD increased from 100 mg/L to 3,350 mg/L within 17.4 min, while TOC reduction did not exceed 18%. BOD improvement was calculated based on the change of molecular weight distribution to the range lower than 380 and the decrease of UV absorbance to the range lower than 2.0 x 10(4) muV in the same molecular weight range. The structural conversion from PVA to biodegradable substances was mainly obtained from hydrolysis and thermal decomposition under sub- and supercritical water conditions. The suitable values of biodegradability (BOD/TOC) for conventional biological treatment methods, 0.78 and 0.79, were obtained in 17.4 min at 350degreesC and in 7.4 min at 400degreesC, respectively. The production of biodegradable substances from refractory pollutants by sub- and supercritical water reactions was the most important factor on the viewpoint of the pretreatment of refractory pollutants.