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A number of techniques have been developed for the measurement of dimethylsulphoniopropionate (DMSP), dimethylsulphide (DMS) and other sulphur species commonly occurring in estuarine sediments. A reaction-based flow injection analysis (FIA) system that was developed enabled the detection and quantification of trace levels of dissolved sulphide, DMSP and DMS precursors, exhibiting detection limits of ca 10⁻⁶ M. An acid reaction step permits the measurement of sulphide species such as S²⁻. A base hydrolysis was found to be selective toward DMSP, whilst a borohydride reaction widened the range of DMS precursors that could be measured. Gas chromatography was used to determine elemental sulphur and DMS. A DMSP lyase that cleaves DMSP to DMS and acrylate was purified and characterised from the green macroalga, that is a dominant species on many estuarine intertidal mudflats. The DMSP-lyase is located both as a soluble and detergent-released protein. The molecular weight of the soluble protein was approximately 25.2 kDa. Three hydrophobic variant forms were isolated from the detergent-solubilized activity. The optimum pH value of this DMSP lyase is pHS.O and the highest measured enzyme activity was observed at 37 °C. However, the influence of NaCl concentration is insignificant. Based on the results of inhibition studies, it is suggested that this DMSP lyase is a metal dependant enzyme but not a thiol-based enzyme. The enzyme activity increased with MgCli concentration but decreased with CaCl; concentration. The Km and Vmax of the soluble DMSP lyase were 0.286 mM and 21.9 nmole DMS/min/mg protein at room temperature, respectively. Green algae were a m^or contributor of DMSP to intertidal sediments of the Tamar estuaiy. High concentrations of DMSP were found in the surface sediments, accompanied by high DMSP lyase activities. The DMSP lyase activities decreased with decreasing DMSP content throughout the sediment depth indicating that there was a strong relationship between them. Due to its very low concentration DMSP is unlikely to be a m^or source of sulphur in the sediments. Elemental sulphur is the m^or sulphur compound in sediments accounting for about 30 to 40 % of the total sulphur which is ca 1.8 wt % of dry weight sediment. Metal sulphides may control the low levels of'dissolved' sulphide in sediment interstitial water observed in this study.