The mean charge states of heavy ions trapped in the earth's magnetosphere are important parameters for distinguishing among magnetospheric theories. No direct measurements have been made at energies greater than ~50 keV. A technique, which uses the E/q dependence of ion drift velocity, is described in this letter. A drift-echo event, which followed a substorm that occurred at 2100-2200 UT at 25 February 1979, is discussed to illustrate the application of this technique. In this event the charge state of helium ions was +2 and CNO ions was >+2 and probably ≥+5. This result implies that the drift-echo particles were from freshly injected solar-wind plasma.