Abstract Kinetic and thermodynamic properties of the reactions of DMSP + X − a3 CH 3 X + CH 3 SCH 2 CH 2 COO − are obtained by time series measurements of methyl halide (CH 3 X) production by the reactions of DMSP with halide ions (X − ). Thermodynamically, DMSP reactions with halide ions to produce methyl halides are not favoured because they are endothermic and Δ G ° > 0 over the range of seawater temperature. Both kinetic and thermodynamic studies suggest that methyl halide production by DMSP + X − can only contribute a very small proportion of the currently estimated oceanic source of methyl halide (