This work shows the results of four Electro-Fenton laboratory tests to reduce the chemical oxygen demand in spent caustic solutions. The treatment consisted of i) a pH reduction followed by ii) an Electro-Fenton process, which was analyzed in this work. The efficiency of the Electro-Fenton process was analyzed as the COD reduction in pure phenol and sulphide solutions and real spent caustic samples. Close to 97% COD removal was achieved for sulphide treatment, and around 81% for phenol treatment. In the real spent caustic sample, 93% COD reduction was obtained. The sulphide content is lowered by both a pH reduction and the Electro-Fenton process, whereas the phenol concentration is not affected by the pH reduction - only by the Electro Fenton process.