A microwave spectrometer and associated apparatus for the detection of unstable free radical species is described. The spectrometer, which operates over a frequency range from 66 to 73 GHz, consists of a tunable Fabry–Perot resonator followed by superhetrodyne detection for high sensitivity at low power levels. The resonator tuning is accomplished by means of a computer‐controlled stepping motor and by a piezoelectric tranducer which also permits the use of source modulation and phase sensitive detection at 100 Hz. The sensitivity of the spectrometer is adequate to permit the detection of rotational transitions with absorption coefficients as small as 2×10−9cm−1. The operation of the spectrometer is demonstrated by the detection of the two free radical species OH and SO. In the case of OH, a number of L‐doubling transitions, previously unobserved, are reported.