Abstract Details are reported for EPR centres induced in various samples after operating a cyclotron resonance type gas discharge inside a vacuum envelope within a microwave cavity. In the case of vacuum crushed GaAs, centres identified as O − 3 were induced on the surfaces at 100 K. The powder parameters were g 1 = 2.016, g 2 = 2.007 and g 3 = 2.000. For sapphire rods at 80 K, a signal was found similar to that previously observed on gamma-irradiated, oxygen exposed sapphire. The evidence enables identification as O − 2 on Al in the surfaces, g∥ = 2.040, g⊥ = 2.001. For quartz at 100 K, the major EPR centres were similar to those from gamma irradiated quartz and appeared to be of bulk nature, unaffected by gas pressure and ascribed to plasma activated impurity defect centres.