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The present work consisted in evaluating the effect of a natural plant reducer, gallic acid (GA), on the discolouration/oxidation of two azo dyes by Fenton processes (Fe 3+ /H 2 O 2  and Fe 2+ /H 2 O 2 ). A kinetic study was performed to better interpret the discolouration data at different temperatures. The 1st-order kinetic model presented the best fit for the experimental data of methyl orange discolouration, while the 2nd-order was better for chromotrope 2R. Due to the addition of GA and the temperature rise, there were increases in discolouration and in the reaction rate constant values. As a highlight, it was possible to verify the reduction of the apparent activation energy ( Ea ) due to the presence of GA. For example,  Ea  for discolouring methyl orange corresponded to 81.5 and 53.6 kJ.mol −1  by Fe 2+ /H 2 O 2  and Fe 2+ /H 2 O 2 /GA, respectively. Thus, it can be inferred that the GA reduces the energy barrier to increase the oxidation of dyes by Fenton processes.

Gallic acid influence on azo dyes oxidation by Fenton processes