Abstract: Distribution and chemical speciation of dissolved inorganic arsenic were examined in Yellow Sea and East China Sea. Results demonstrated that: (1) both As(III) and As(V) were detected, with As(V) dominated at 40% stations of surface water and 51% stations of bottom water; (2) influenced by the exchange of fresh and sea water, the type of surface sediment and the transport of various water masses, large values in surface water were observed along the coastal region and in the same latitude of Changjiang River Estuary and Hangzhou Bay, and in bottom water found in the southern area where the Taiwan Warm Current and Kuroshio Current influenced; (3) As(III) behaved non-conservatively in Changjiang River Estuary. Man-derived inputs cause substantial positive deviation from the theoretical dilution. The negative correlation of As(V) to salinity in surface water suggested that it behaved conservatively during the transportation along Changjiang River Estuary. While, the occurrence of As(V) up to the linear fit in bottom water indicated the eventual transfer from dissolved phase to particulate phase, which was impossible to be determined without the knowledge of arsenic level and speciation in suspended particulate matter. Further study is needed on the arsenic source/sink relationships based on the distribution profiles.