Dimethylsulphoniopropionate (DMSP), an algal compatible solute, has for many years been considered to play a key role in dimethylsulphide (DMS) production, influencing the concentrations of DMS in sea water available to be transferred to the atmosphere. However, in recent years it has been shown that dimethylsulphoxide (DMSO) can also be produced directly within the cells of marine phytoplankton. The exact role DMSO plays in cells is still subject to debate, but it is thought that it may act as an antioxidant or cryoprotectant. Whatever the reason, it has been suggested that release through algal mortality and permeative loss of DMSO from cells may contribute to dissolved DMSO concentrations and as such this pathway must also be considered an important component of DMS biogeochemistry. Experiments were conducted to investigate the intracellular concentrations of DMSO and the ratio of DMSP:DMSO in a range of phytoplankton species and in natural samples. Results indicate that prymnesiophytes and dinoflagellates are the main producers, generating relatively higher concentrations of particulate DMSO than diatoms. Results from both laboratory and field experiments show that there is a strong relationship between DMSOp and DMSPp, with DMSO generally representing between 10 and 20% of the intracellular sulphur pool. Field data also indicates that dissolved DMSO concentrations in surface waters were not significantly correlated with those for particulate DMSO, but were significantly correlated with DMS concentrations.