This note discusses sources of error in the use of charged-particle analyzers to determine the ionic species and energies in plasma streams, with particular reference to the widely used electrostatic instrument. Space-charge expansion within the analyzer leads to an upper limit on the size of the entrance aperture. This limit, which depends on the ion energy and density, is less than the proper Debye shielding distance that is often considered a sufficient limit. For ion energies below a few hundred eV and densities above 1010 cm−3 the aperture requirement is impractical. Therefore the possibility of reducing the plasma density by means of single apertures or multi-hole screens is considered. These considerations are supported by measurements on plasma gun outputs. The best criteria for proper operation of the analyzer are that the integral over the output spectrum equal the number of incident ions and that the output intensity be proportional to the area of the entrance hole.