Proton nuclear polarization is observed in the photoreaction of aromatic vitro-compounds with 2,6-di-t-butyl-phenols. The polarization is due to a reversible abstraction of the phenol hydroxy-hydrogen by the excited triplet nitro-compound. The occurrence of polarization depends on the triplet energy of the vitro-compound and on the Oâ H bond dissociation energy of the phenol. Observation of polarization forms the basis for the estimation of the Oâ H bond dissociation energy of ArNO2H· radicals.