To clarify the mechanisms of the Okhotsk Sea intermediate water formation, water samples collected in the Okhotsk Sea and neighboring western North Pacific were analyzed for Δ18O as well as salinity and other routinely measured components. The Δ18O-salinity relation in the Okhotsk Sea was markedly different from that in the western subarctic Pacific feeding the Okhotsk Sea. The intermediate water of the Okhotsk Sea was less saline than that of the western subarctic water at the same density but was more saline at the same Δ18O value, especially in the density range 26.5<&sgr;&thgr;<27.0. This could be due to mixing of dense shelf water formed during sea-ice formation with neighboring water. The amount of fresh water removed by the sea-ice formation was estimated from the salinity anomaly of the Okhotsk Sea intermediate water. The calculated amounts of fresh water removed from the Okhotsk Sea intermediate water corresponded to the amount of sea ice formed for 0.6--3.8 years in the Okhotsk Sea. This suggests that the residence time of the Okhotsk Sea intermediate water is a few years. ¿ 2001 American Geophysical Union <