Fenton reactions are widely known and have been employed to degrade different pollutants in various environmental matrices. However, some controversies related to the Fenton mechanism remain unresolved. Although not comprehensively understood, kinetic sigmoidal profiles with an initial induction period have frequently been reported for Fenton systems. A thorough kinetic study was carried out at different temperatures involving Fenton reactions promoted by Fe3+ and Fe2+ and under the influence of radical scavengers. Catalysis supported by Fe2+ displayed the usual pseudo-first-order profile, while Fe3+ catalysis showed a sigmoidal pattern. The sigmoidal profiles were correlated to a pseudo-elementary autocatalytic mechanism developed by Finke and Watzky (F-W). The use of the F-W equation indicates that a certain amount of oxidative species must be generated to promote the substrate degradation. This correlation is a breakthrough in the understanding of Fenton catalysis. (C) 2018 Elsevier Inc. All rights reserved.