This paper demonstrates that CTD data from autonomous profiling floats can be used to detect basin-scale changes of water properties on decadal, and potentially shorter, timescales. In the western Indian Ocean at 32¿S, the properties of Subantarctic Mode Water have changed such that between 1987 and 2002, salinity has increased by 0.04 at certain temperature levels. Although in this case detailed and high-accuracy data are available from a research cruise, we demonstrate that floats have the following value: real-time float data are of sufficient accuracy to reveal changes in ocean properties on timescales of a decade or less; floats achieve spatial and temporal coverage not possible with research ships, providing vital supporting evidence for the interpretation of cruise CTD data. Floats therefore provide a mechanism for monitoring changes during the interval between dedicated research cruises and for identifying areas in which further cruises are likely to be fruitful.