AbstractChlorocyclohexane does not react in the dark with diphenylphosphide ions in liquid ammonia, but it reacts under irradiation to give the substitution product. Bromocyclohexane reacts slowly in the dark, but this reaction is strongly accelerated by light, and inhibited byp‐dinitrobenzene. 3‐Bromo‐2‐tetrahydropyranyl allyl ether (used as a radical probe) reacts with diphenylphosphide ions under irradiation to give both the straightforward and the cyclized substitution products. This photostimulated reaction is also inhibited byp‐dinitrobenzene. It is suggested that the chlro‐bromocyclohexanes and related compounds react under irradiation by the SRN1 mechanism of nucleophilic s