An apparatus incorporating a vacuum microbalance that can make simultaneous measurements of both the mass of a sample and its magnetic susceptibility is described. The Faraday technique is employed to measure the susceptibility, but the field gradient is provided by a set of coils rather than the shaped pole faces commonly used. Because the gradient can be reversed in less than one second, by switching the coil current, the susceptibility can be measured even when sample mass is changing rapidly. This allows one to correlate changes in susceptibility with mass during in situ chemical reactions. The system also includes a variable temperature probe that can vary sample temperature in a controlled manner through the range of 4.2 K to over 800 K in a few minutes. Measurements made during the sulfiding of a CoO-MoO3-Al2O3 catalyst are presented as an example of the use of the apparatus.