AbstractElectron spin resonance is observed of photosensitive 2S1/2‐state divalent gallium, indium and thallium ions in ZnS. The ESR spectra show a large hyperfine interaction which may be greater than the energy of the microwave quantum. This suggests that the unpaired electron is highly localized at the impurity site. In contrast, the orbit of the unpaired electron of the shallow aluminium and halogen donors in ZnS exhibit considerable delocalisation.