A mass spectrometer which can be used to measure relative ion concentrations in a multi‐ion component plasma has been designed for use in a strong (1–4‐kG) uniform magnetic field. The spectrometer features an acceleration region which accelerates thermal ions through a series of three tantalum electrodes at a 30° angle to theBfield, and a collection region in which ions are selectively collected, depending on the size of their gyroradii, by a cylindrical collector. Relative ion concentrations are determined from measurements of the collector current as a function of accelerating voltage. Results obtained using this instrument in aQ‐machine device operated with a two‐ion (Cs+/K+) component plasma are presented.