Magnetic separation is routinely used in the processing of titanium minerals, and the efficiency of these operations can be determined by measuring the magnetic properties of the process streams. The valuable components in the feed to a titanium minerals plant have specific magnetic susceptibilities ranging from -10-9 m3 kg-1 (zircon) to 10-6 m3 kg-1 (ilmenite). However, in an industrial environment, in which the mineral temperature may range from 20 to 120C, it is difficult to measure susceptibilities lower than about 10-7 m3 kg-1. Hence laboratory testing of spot samples from magnetic separators is currently required to obtain accurate performance data. This is an inherently slow process and where feed grades are variable, optimum performance of magnetic separators is generally not achieved. This paper describes the development of an inductance-based instrument for measuring very low levels of magnetic susceptibility on-line. The signal from the instrument can be used for process control purposes to maintain optimum magnetic separator performance. A prototype instrument has been installed in the zircon scavenger circuit in an Australian titanium minerals plant. The circuit consists of a roll magnetic separator treating a feed consisting mainly of monazite and stained zircon. The monazite-rich magnetic fraction is returned to the mine site for disposal, while the non-magnetic fraction is recycled to the zircon wet circuit. Splitter positions in the separator are adjusted when the final zircon product fails to meet specification, or the magnetic fraction contains more than 30 per cent zircon. Weekly composite grain counts show that sub-optimal operation of the magnetic separator results in significant zircon losses into the magnetics fraction. Using the instrument, the zircon content of this stream can now be accurately determined from the measured magnetic susceptibility, provided a correction for the effect of mineral temperature is applied using the Curie - Weiss relationship.