In the Indian Ocean sector of the great ocean conveyor scheme the North Atlantic Deep Water (NADW) replacement is realized by the transformation of the sinking Circumpolar Deep Water (CDW) from the Southern Ocean to a prevailingly upward flow in the northern Indian Ocean. This water -mass transformation scenario has been studied by using hydrographic data including potential temperature, salinity, dissolved oxygen, and nutrients in a water - mass mixing model and by calculation of dianeutral velocity. The model comprises a mixing system of three major deep water masses, NADW, CDW, and North Indian Deep Water. This third water mass is introduced as a virtual water mass to represent the aged CDW. The assumption is feasible as the artificial water mass can be eliminated on the deepest &sgr;N=28.12 neutral surface, characterized by isopycnal mixing. The experiment is fulfilled when a conservative variable, initial phosphate PO4¿, is introduced and all conservative parameters are used in the mixing scheme. The &sgr;N=28.12 surface provides a major isopycnal transition path of CDW to the northern Indian Ocean, and thus CDW is transformed to an upward flow. Dominant dianeutral upwelling is detected on all four neutral surfaces north of 30 ¿S, showing increasingly strong dianeutral velocity toward shallower surfaces. With the water - mass mixing pattern and spreading path, dianeutral circulation, and dynamical information of the acceleration potential (10 m2 s-2) mapped on neutral surfaces a schematic of deep water circulation and ventilation of the Indian Ocean emerges from this study. The contribution of NADW is largely limited to the southwest Indian Ocean. The northward CDW is found mainly in two paths: one in the western Indian Ocean (apparent between 2000 and 3000 m) through the western Crozet and Madagascar Basins, and farther north into the Somali Basin and Arabian Basin and another in the Central Indian Basin (apparent below 3000 m where ridge-blocking becomes effective in the west and NADW salinity reaches a maximum). While the major path switches from west to the central basin in the deeper layer (3000--3500 m), the flow in the western Indian Ocean reverses southward. North of the Southeast Indian Ridge, the deep path bifurcates, from which point the eastern branch feeds the deep western boundary flow in the West Australian Basin. Except for part of the transformed CDW, which is contributed to the upward flow in the north, the return flow of CDW is found on the eastern side of these basins. ¿ 2000 American Geophysical Union