This work aims to study the treatment of a dye bearing effluent from an agate-processing plant by sodium hypochlorite (NaClO) oxidation. The work includes effluent characterisation, chemical oxidation experiments and evaluation of degradation products by Gas Chromatography coupled to Mass Spectroscopy/Ion Trap (GC/MS/IT). The effluent presents a very intense colour (lambda(max) approximate to 550 nm), low surface tension and some organic content. Colour removal can be carried out in a few minutes using 1.3 ml/l of NaClO at pH 7.0. Organic dyes are not completely oxidised by NaClO and many degraded compounds were found in the treated effluent, including organochlorides. The use of chlorine in the effluent treatment is discussed in terms of pollutants degradation and environmental hazards. (C) 2002 Elsevier Science B.V. All rights reserved.