The Low Energy Ion Mass Spectrometer (IMS-LO) was designed to measure the energy and mass spectra in the energy/Q range 0.1 - 32 keV/Q and in the mass range 1.0 - 32 amu/Q. The design was very similar to the spectrometers of the Lockheed group on board the SCATHA and S3-3 satellites. Two identical instruments, IMS-LO-1 and IMS-LO-2 were used: one looking at 45 deg and the other at 75 deg with respect to the spin axis of the satellite. The field of view was 5 deg, conical. Each IMS was actually a triplet of spectrometers, covering the three ranges, 0.1 - 0.6 keV/Q, 0.7 - 4.5 keV/Q, and 5 - 32 keV/Q. In turn, each energy range was covered by 15 discrete energy steps, with a duration of 2.084 s at each step. Each of the 6 (3 x 2) spectrometers consisted of a Wein velocity filter, followed by an electrostatic energy analyzer (EEA) and then by a channel multiplier. The geometric factor at any energy was about 4.E-4 sq cm-sr. At each energy step, the velocity filter sequentially sampled for 2.084 s the masses in 32 mass steps. The instruments could also be operated in a LOCK submode, in which the mass was held fixed and the range of 0.1 - 32 keV/Q was swept in steps during a total duration of 1.024 s. The two IMS-LO systems could be independently commanded to operate in the SWEEP or the LOCK submode. Four fixed-energy electron detectors were attached to each IMS-LO system, and together the eight electron detectors covered the energy range 50 eV - 25 keV, in 512 ms.