For the southeast Indian Ocean, isentropic analysis of salinity and oxygen distributions on the 27.20 and 26.80 σt surfaces showed that mixing of sub-Antarctic intermediate water with poorly oxygenated bottom water· of Arabian Sea origin was confined to the region west of about 90° E , east of which there was sinking and movement of oxygenated waters from tropical and subtropical convergences and from the Banda Sea. The seasonal change of properties on the 27.20 and 26.80 σt surfaces off Fremantle gave some indication of the seasonal flow paths of these different waters.