The testing of iron ores for beneficiation by magnetic separation is normally done in laboratory separators of various types and the results are reported in terms of the variables, such as magnet coil current, rotor speed and gap width, of the instruments used. Such testing is empirical, and the results can be reproduced only in a machine precisely similar to the test machine. A less empirical method of testing is clearly desirable, and a project was therefore initiated to develop such a method. The basic physical property of minerals on which their magnetic separation depends is the magnetic susceptibility. The magnetic investigation of an iron ore should therefore aim to characterise the ore in terms of the susceptibilities of its components or of the fractions into which it is desired to separate the ore. In addition, it is necessary to show how a knowledge of these susceptibilities can be applied to the treatment of the ore in industrial separators.