The Solar Wind Ion Mass Spectrometer (SWIMS) measures every few minutes the mass and energy of most solar wind ions and many isotopes with velocities in the range from 200 - 1500 km/s for He to 200 - 500 km/s for Fe. Abundances of rarer isotopes and ultra-heavy ions (A > 60) are determined every few hours. Mass resolution is less than one percent. The instrument consists of a versatile deflection system followed by a time-of-flight (TOF) telescope. Solar winds ions enter a wide angle three-chamber deflection system acting as an energy-per-charge passband filter, the range of the passband being adjustable by change of the deflection voltage. The entry aperture field of view is 50 deg. x 60 deg. Ions within the passband enter the mass analyzer section of the instrument by passing through a thin carbon foil, from which the incident ions emerge mainly as neutrals or singly charged ions. The start signal for TOF analysis is generated by secondary electrons ejected from the tin foil and deflected into a microchannel plate (MCP) assembly. The ions are then deflected in an electrostatic square potential and stop in a large area MCP assembly, thereby triggering a stop signal for TOF determination of velocity. SWIMS telemetry includes pulse height analysis (PHA) and counting rate data for ion events, and housekeeping data. Readout of five PHA events per second requires 64 bits per event, while rates add another 180 bits/s. Another 5 bits/s is required for housekeeping and status data. The SWIMS experiment manager is Prof. G. Gloeckler of the University of Maryland.