Autonomous CTD profiling float data from Argo between 2000 and 2004 have been used to study the properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in relation to local winter mixed layer properties and frontal regimes in the South Indian Ocean. By calculating noise variance at the tightest part of the -S curve, the accuracy of salinity in the selected Argo data set has been estimated to be better than 0.01 psu. The basin-wide and seasonally unbiased sampling of the float array shows that South Indian Ocean SAMW is more continuous in geographical and density space than previously observed. Its density and thickness distributions are related to the winter mixed layer properties in the local Subantarctic Zone, with progressively denser and thicker SAMW to the east. The salinity minimum associated with South Indian Ocean AAIW was centered on σ = 27.2 kg m−3. As observed by previous hydrographic surveys, fresh AAIW is injected northward into the subtropical gyre in mid-basin, at about 65–85E. Argo profiles show that this northward flow extends to the northern extent of the study region at 15S. In the latitude band 30–40S and east of 55E, meridional salinity gradient on density surfaces in AAIW is the smallest in the South Indian Ocean. Hence that location is the best place in the South Indian Ocean to detect integrated decadal changes in the salinity minimum of AAIW.