Abstract: Distributions of dissolved and particulate dimethylsulfoxide (DMSOd and DMSOp) were studied in the surface waters at different depths in the East China Sea and the Yellow Sea from December 2011 to January 2012, to explore their sources and influencing factors. The concentrations of DMSOd and DMSOp in the surface waters were (10.10±7.54) and (8.72±7.80) nmol/L, respectively, which were clearly affected by phytoplanktons and abundance in the area. The vertical profiles of DMSOd and DMSOp were characterized by high concentrations in the upper mixed layer (3-20 m). No relationship was found between DMSOd and DMS, whereas a significant correlation was observed between DMSOd and DMSOp (R=0.442, n=41, P<0.006). The result suggested that DMSOd mainly originated from the diffusion of intracellular DMSOp, rather than from the oxidation of DMS (photochemical and biological oxidation). In addition, the positive linear correlation between DMSOp/Chla and salinity (R=0.532, n=46, P<0.004) indicated that changes of salinity might influence the phytoplankton taxonomic composition and thus DMSOp concentration.