The wet oxidation process (water; oxygen and elevated temperature) was investigated under alkaline conditions for fractionation of hemicellulose, cellulose, and lignin from wheat straw. At higher temperature and longer reaction time, a purified cellulose fraction (69% w/w) was produced with high enzymatic convertibility to glucose (66% w/w). At 185 degrees C, nearly three times more hemicellulose was solubilized than at 150 degrees C. Optimum conditions for hemicellulose solubilization and cellulose convertibility were for 60 g l(-1) straw: 185 degrees C: 6.5 g l(-1) Na2CO3, and 12 bar O-2 with a 15-min reaction time. Under these conditions, 55% of the lignin and 80% of the hemicellulose were solubilized, while 95% of the cellulose remained in the solid fraction. At 185 degrees C, the reaction kinetics was of pseudo first-order. The rate constant for hemicellulose solubilization was higher than that for lignin, whereas the rate for cellulose was very low. The cellulose recovery (95-100%) was significantly higher than that for hemicellulose (60%). At temperatures above 185 degrees C, recoveries decreased due to increased degradation. Only half of the COD-content could be accounted for by saccharides and carboxylic acids; hence, a significant proportion of reaction products remained unidentified.