A comparison between two analytical techniques commonly used for the determination of sulphur concentrations in natural water samples is reported. Significant differences in the sulphur values obtained using inductively coupled plasma atomic emission spectrometry and ion chromatography are described. The contribution of a sulphur fraction (presumed to be organic S) not detected by ion chromatography is identified. This contribution varies considerably between individual samples and is highly correlated to the concentration of dissolved organic carbon. The results are explained in terms of an organic S component which is of environmental importance.