The non-catalytic wet air oxidation (WAO) of olive mill wastewaters was investigated. The effect of operating conditions, such as initial organic loading (1,000 and 4,500 mg/L COD), reaction temperature (140 and 180 degrees C), treatment time (1 and 4 h), initial pH (4.8 and 7) and the use of 500 mg/L H(2)O(2) as an additional oxidant, on treatment efficiency was assessed implementing a factorial experimental design. Of the five parameters tested, the first two had a considerable effect on COD removal, while treatment time was of no significance implying that all oxidation reactions occur during the first hour of treatment. Although the level of mineralization was generally moderate, this was accompanied by nearly complete total phenols and color removal. The analysis was repeated at more intense conditions, i.e. initial COD up to 8,000 mg/L and reaction temperature up to 200 degrees C; at this level, none of the studied effects were important. However, at optimal experimental conditions (i.e. 180 degrees C, 1 h treatment and initial COD of 8,100 mg/L) WAO yielded 34, 94 and 74% removal of COD, total phenols and color respectively. Moreover, ecotoxicity to V. fischeri was slightly reduced after 2 h of treatment at the above conditions.