<p>In the processing of agates, the gemstone pieces are submitted to dyeing processes. One of the dyeing processes applied is with Rhodamine B, which provides a reddish-pink colouration. The practice generates a wastewater containing residual amounts of the dye and ethylic alcohol with high polluting potential. Thus, the objective of this paper is to investigate the chemical parameters of the Fenton´s oxidation process to treat this effluent. The main parameters evaluated were [H₂O₂]:[Fe²⁺] molar ratio and the concentration of the reagents H₂O₂ and FeSO₄·7H₂O. The wastewater contained a concentration of 772 mg L⁻¹ of Rhodamine B and 3% of ethanol in its composition, giving the effluent a high colour, organic load, and toxicity. In the Fenton reaction treatment, 11.1 g L⁻¹ of ferrous sulphate and 20 mL L⁻¹ of oxygen peroxide (in a molar ratio [H₂O₂]:[Fe²⁺] of 7.5:1) were defined as the best dosage, which allowed complete alcohol removal, an average absorbance reduction of 99.9% at 554 nm, a TOC reduction of 93.3%, an increase in surface tension from 58.9 to 64.4 mN m⁻¹ and a toxicity factor (TF) decrease from 526 to 16 with respect to the organism <i>Daphnia similis</i>. The Fenton process combines oxidation, coagulation and air stripping mechanisms that substantially reduces pollutants present in this effluent, at a rougher stage. The results obtained are useful, both for the gemstone sector and others that make use of Rhodamine B dye. </p>