Nowadays the potential of photo-Fenton process for decontamination of biologically persistent wastewater is widely recognized. In this process, hydrogen peroxide consumption could be considered as one of the factors that considerably affects costs. This highlights that an automatically hydrogen peroxide dosing system to keep the concentration optimized throughout the process would be a valuable tool for industrial wastewater remediation plants. Automatic H(2)O(2) dosing during photo-Fenton would require a precise online measuring device. Today, several expensive and unreliable in complicated wastewater hydrogen peroxide probes are commercially available. This study focuses on considering dissolved oxygen concentration (DO) as a reaction stage indicator related to H(2)O(2) concentration and consumption during photo-Fenton wastewater treatment. First, specific experiments designed to find out the DO-H(2)O(2) related behavior during photo-Fenton at different possible operating conditions were carried out with manual addition of H(2)O(2). Afterwards, a following set of experiments was performed with automatic H(2)O(2) dosing according to DO concentration maintaining this parameter between desirable set points during the photo-Fenton reaction. The obtained results demonstrated the potential for making use of the DO profile for monitoring photo-Fenton, automatically dosing H(2)O(2) and determining the end point of the treatment. The active participation of oxygen in the first stages of the process by Dorfman-mechanism was confirmed. (C) 2010 Elsevier B.V. All rights reserved.