An analysis of historical oxygen data provides evidence on the water exchange between the South China Sea(SCS)and the Pacific Ocean(PO).In the vicinity of the Luzon Strait(LS),the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 rn(intermediate layer),while the situation is reversed above 700 rn(upper layer)and below 1 500 rn(deep layer).The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers,supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical.Within the SCS basin,the oxygen distribution indicates widespread vertical rnovernent,including the upwelling in the intermediate layer and the downwelling in the deep layer.