The objective of this research was to investigate the decomposition and decoloration of textile wastewater by hydrothermal treatment. We studied the hydrothermal oxidation of methyl orange aqueous solution using a flow-type reactor packed with MnO2 catalyst. Hot steam (vapor) and subcritical water (liquid) were used in the reactor as the decomposition media at the reaction temperature and pressure of 200-300 degrees C and 1 or 10 MPa, respectively. When methyl orange solution with 300 mg/L of total organic carbon (TOC) was used, nearly complete decomposition of methyl orange was realized in hot steam at 300 degrees C,1 MPa, 10 s, and 1.5 oxygen supply ratio, i.e., a much higher TOC decomposition rate was obtained at a lower reaction pressure and shorter residence time than those obtained with subcritical water oxidation. The kinetic modeling of hydrothermal oxidation of the model wastewater containing methyl orange was also investigated. The kinetic model to calculate TOC conversion at a given residence time was proposed based on the regression from the complete set of data. In the case of hot steam oxidation, a second-order kinetic model was used to describe the TOC conversion. However, a two-step first-order kinetic model (a rapid first reaction followed by a slow second reaction) was needed to describe the kinetics of subcritical water oxidation. Both reaction rate expressions were successfully used to estimate the TOC concentrations at any residence time at 90% confidence level. (C) 2015 Elsevier B.V. All rights reserved.