Planar laser induced fluorescence (PLIF) technique was used to investigate concentration profiles and reaction progress of a turbulent reactive mixing process in a baffled stirred tank reactor. Rushton turbine, up-pumping pitched blade turbine (PBTU), and down-pumping pitched blade turbine (PBTD) were used to study the effect of mixing on the fast Fenton reaction progress. For determining the reaction progress, a criterion was developed, which was based on obtaining the number of mixed pixels with a defined uniformity percentage for the entire reactor area. It was found that impeller type, impeller clearance, and reactant injection location significantly altered concentration profiles and reaction progress. The reaction time scale along with the micro- and macromixing time scales were determined and it was concluded that the system performance was controlled by macromixing in the tank. Mean velocity field of the impellers was obtained by particle image velocimetry (PIV) technique and reasonably validated the concentration distribution results of PLIF technique. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.