The microwave cavity method for the measurement of various parameters of a cold plasma in the presence of a static magnetic field is examined. Emphasis is placed on the determination of the limits of validity of the perturbation theory for various mode configurations of a cylindrical cavity coaxial with a plasma column and coaxial with the static magnetic field. The classes of modes examined are those which in the absence of the magnetic field reduce to the TM0m0, TM1mn, TE1mn, and TE0mnmodes. For the TM0m0and TE0mnmodes, exact expressions for the cavity frequency shifts are obtained. These expressions are then expanded in appropriate power series to obtain the limits of validity of the perturbation method. For the TE0mnmodes the perturbation theory must be modified to account for the polarization of the plasma. In the absence of a magnetic field, the TM1mnas well as the TE1mnmodes are degenerate in their resonant frequencies. The presence of a magnetic field removes the degeneracy and causes the resonant frequency to be double‐valued. An experimental test of the validity of the perturbation method can be had by comparing the two resonant frequencies. The relation is given between the characteristics of the various modes and the propagation of plane waves in infinite uniform plasmas.