The rate of unipolar arcing to a Ti sample which had been activated by 7 MeV protons impinging on part of it after passing through an N2 atmosphere has been observed to be considerably reduced compared to the rate of arcing on the unirradiated part of the sample. This observation, first made in the DITE Tokamak, was confirmed by laboratory arcing studies on Ti and stainless steel specimens. It has been shown to be the consequence of irradiation through a gaseous atmosphere and to be avoidable by irradiationin vacuo.