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When sulfur is measured in natural water samples by means of inductively coupled plasma-atomic emission spectrometry (ICP-AES), its emission intensity is mainly due to sulfate. Because of its low concentration, the sulfur emission coming from sulfide can be considered negligible. In the present work, it is shown that H2S generated by previous acidification of the sample enhances the sulfur signal emission as compared to the signal that is found when the water sample is analyzed in the absence of acid. The generation of hydrogen sulfide (H2S) takes place in a coil in which sulfide reacts with a hydrochloric acid solution. When this procedure is used, sulfur emission intensity is due to both sulfate and sulfide. The contribution of sulfide is obtained by subtracting the emission intensity measured straight from the water sample from that measured in the acidic mediumThe authors would like to thank the Spanish Education Ministry (Project Ref. PETRI95-0980-OP and Project Ref. CGL2004-05963-C04-03) and the Vice-Presidency for Research of the University of Alicante for the financial support for this work. M. Colon gratefully acknowledges a grant from the Spanish Ministry of Science and Technology (Ref. BES-2005-6962)