A simple electron spin resonance (ESR) spectrometer for the detection of organic free radicals is described. The salient features are: (1) homodyne balanced mixer detection, (2) improvement of the magnetic field homogeneity by means of two pairs of current shims, and (3) new tuning procedure independent of the resonance spectrum itself, making use of the klystron‐stabilization error signal. At 3200 gauss the magnetic field homogeneity is better than 2:105over a sample volume of 0.25 cm3. The sensitivity is such that 2×1013spins of the free radical DPPH can be detected.