Treatment of olive mill wastewater (OMW), which has an important environmental pollution potential in Mediterranean countries, was studied in sub- and supercritical water at 10-30 MPa, 400-650 degrees C in a tube reactor using hydrogen peroxide as an oxygen source. The initial total organic carbon (TOC) concentration of the OMW was in the range of 23.64-291.86 mmol L(-1), and the initial oxygen concentrations were between 147 and 588 mmol L(-1) at the process conditions. The results demonstrated that the treatment efficiency of the OMW increased significantly with temperature, but decreased with pressure. Supercritical water oxidation (SCWO) process decreased the TOC up to 99.96% in very short reaction times (at residence times of 5-30 s). According to the wastewater and oxidant concentrations, the global reaction rate expression which consists of the thermal decomposition and the oxidation reaction was regressed from the complete set of data. As a result of regression analysis, the first-order reaction rate for the thermal decomposition of the OMW was determined with the activation energy of 40.36 (+/- 0.46) kJ mol(-1) and the pre-exponential factor of 14.09 (+/- 1.05) s(-1), and the oxidation reaction rate orders for the TOC and the oxidant were 1.02 (+/- 0.031) and 0.89 (+/- 0.054) with the activation energy of 33.24 (+/- 0.09) kJ mol(-1) and the pre-exponential factor of 0.214 (+/- 0.5) L(0.91) mmol(-0.91) s(-1) in a 95% confidence level. (c) 2008 Elsevier B.V. All rights reserved.