Indigo carmine is a blue synthetic dye widely used in textile industry, as a food additive, in pharmaceutical industries, and as pH indicator. The use of this dye can leave to contamination of waste water and environment, damaging aquatic fauna and flora. An alternative for the treatment of water contaminated by dyes would be the degradation through oxidative processes. In this work, the indigo carmine degradation through a heterogeneous Fenton process using fly ash, an industrial waste, as Fe(II) source, was evaluated.. The dye final concentration was quantified by ultraviolet/visible light absorption spectroscopy. The optimal reaction conditions were determined using factorial design. The results showed that fly ash is able to remove approximately 42 % of the indigo carmine dye from solution without any pre-treatment.