This thesis describes the construction and calibration of a cycloidal type ion mass spectrometer (CIMS) for analysis of space plasmas. Crossed electric and magnetic fields are used to force incoming ions to execute cycloidal trajectories with a net displacement proportional to the mass per unit charge of the ions. The magnetic field is produced by a permanent magnetic while the electric field is varied to select which mass is to be detected. A funnel type channeltron is used as the detector. The main feature of the CIMS is the energy and entrance angle independence of the selection process. In order to determine the real limits of this feature, an Ortec R. F. ion source, capable of producing high fluxes of heavy ions (up to 4 amu) over a wide energy range, was used to calibrate the instrument. The derivation and determination of the mass resolution R=mAm, the energy range, and geometric factor is discussed. The can detect ions in the low keV range with a mass resolution of R = 8, and an •6 9 estimated average geometric factor G = 1x1^-6cm^2sr.