NTO (5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one)is shown to form aphotochemical free radical in alkaline aqueous solution containing nitromethane (CH3NO2). The NTO free radical can also be produced by chemical reduction of NTO with hydroquinone in alkaline solution. Details of radical formation and assignment of the NTO radical to a nitro anion radical, having the NTO ring intact, are presented.