A chemoreduction-purge-and-trap gas chromatographic method has been developed for the deter-mination of trace dimethylsulfoxide (DMSO) in seawater. In the analysis procedure, DMSO was first reduced to dimethylsufide (DMS) by sodium borohydride and then the produced DMS was analyzed using the purge-and-trap technique coupled with gas chromatographic separation and flame photometric detection. Under the optimum conditions, 97% DMSO was reduced in the standard solution samples with a standard deviation of 5% (n=5). The detection limit of DMSO was 2.7 pmol of sulfur, corresponding to a concentration of 0.75 nmol/L for a 40 ml sample. This method was applied to determine the dissolved DMSO (DMSOd) and particulate DMSO (DMSOp) con- centrations in the surface seawater of the Jiaozhou Bay, and the results showed that the DMSOd and DMSOp concentrations varied from 16.8 to 921.1 nmol/L (mean:165.2 nmol/L) and from 8.0 to 162.4 nmol/L (mean: 57.7 nmol/L), respectively. The high concentrations of DMSOp were generally found in productive regions. Consequently, a significant correlation was found between the concentrations of DMSOp and chlorophyll a, suggesting that phytoplankton biomass might play an important role in controlling the distribution of DMSOp in the bay. Moreover, in the study area, the concentrations of DMSOd were significantly correlated with the levels of DMS, implying that the production of DMSOd is mainly via photochemical and biological oxidation of DMS. A chemoreduction-purge-and-trap gas chromatographic method has been developed for the deter-mination of trace dimethylsulfoxide (DMSO) in seawater. In the analysis procedure, DMSO was first reduced to dimethylsufide (DMS) by sodium borohydride and then the produced DMS Under the optimum conditions, 97% DMSO was reduced in the standard solution samples with a standard deviation of 5% (n = 5). The detection was performed using the purge- and-trap technique coupled with gas chromatographic separation and flame photometric detection. limit of DMSO was 2.7 pmol of sulfur, corresponding to a concentration of 0.75 nmol / L for a 40 ml sample. This method was applied to determine the dissolved DMSO (DMSOd) and particulate DMSO (DMSOp) con- centrations in the surface seawater of the Jiaozhou Bay, and the results showed that the DMSOd and DMSOp concentrations varied from 16.8 to 921.1 nmol / L (mean: 165.2 nmol / L) and from 8.0 to 162.4 nmol / L (mean: 57.7 nmol / L) high concentrations of A significant correlation was found between the concentrations of DMSOp and chlorophyll a, suggesting that phytoplankton biomass might play an important role in controlling the distribution of DMSOp in the bay. Moreover, in the study area, the concentrations of DMSOd were significantly correlated with the levels of DMS, implying that the production of DMSOd is mainly via photochemical and biological oxidation of DMS.