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Flow injection apparatus was designed based on the use of selective reaction chemistry and sulphur specific flame photometric detection. The performance of two reaction systems was assessed using seven synthesised dimethylsulphonium compounds (dimethylsulphonio propionate (DMSP), dimethylsulphonio-2-methyl propionate, S-methyl methionine (SMM), dimethylsulphonio acetate, dimethylsulphonio butanoate, dimethylsulphonio pentanoate, dimethylsulphonio choline). Base hydrolysis was found to be selective towards dimethylsulphonio propionate, whilst sodium tetrahydridoborate (borohydride) gave a virtually equal response to all seven compounds. Two HPLC instruments were developed based on the reaction chemistries and detection systems employed in the flow injection apparatus. The chromatographic separation of the seven synthesized DMS precursor compounds on both anion- and cation-exchange media was evaluated for the identification of DMS precursors. This apparatus provides the basis for the first reliable identification of DMS precursor compounds at trace levels. The 3σ detection limit employing base hydrolysis is 1.29 x 10⁻⁷ mol(DMSP) dm⁻³ (200 μl injected), corresponding to an absolute injected mass of 3.5 ng (DMSP). Macroalgae, microalgae, flowering plants, barley and cyanobacteria were analysed for DMS precursor compounds. DMSP was identified as being present in a wide range of flora at concentrations of between 2.9 μmol kg⁻¹ fresh weight (FW) and 29.4 mmol kg⁻¹ FW. The instability of DMSP towards enzymatic and chemical breakdown requires particular attention to be given to sample collection and storage. An extraction method was developed involving 2% HC1 to deactivate DMSP- consuming enzymes and to slow chemical hydrolysis. The results constitute the first direct sulphur specific measurements of DMSP at trace levels. In macroalgae, levels ranged from 0.0010 to 29.4 mmol(DMSP) kg⁻¹ FW. For plankton levels between 0.82 to 158 μmol(DMSP) mg(chl'a')⁻¹ were recorded. SMM was found in cabbage and parsley (74.32±7.00 and 16.11 ± 0.78 μmol kg⁻¹ FW respectively) and these results constitute the first direct measurements of SMM at trace levels.