The summer distribution of sea ice meltwater and river runoff and the surface circulation in Foxe Basin, Hudson Bay, and Hudson Strait have been studied by oxygen isotope techniques. On the basis of oxygen isotope and salinity relationships, the reduction of surface salinity in summer in Foxe Basin is predominantly due to sea ice meltwater. River runoff and sea ice meltwater contribute equally to the summer surface layer in northern Hudson Bay. The isotopic data clearly show the flow of Foxe Basin water through Roes Welcome Sound into northwestern Hudson Bay, but no evidence for this outflow is found in the passages connecting Hudson Bay and Hudson Strait. Oxygen isotopes indicate that recent river runoff is concentrated in a narrow band along the south side of Hudson Strait. The wide variety of relationships between oxygen isotope ratios and salinity is explained by the known ice dynamics and the seasonal variability predicted from ice melting and freezing models.