[14C]Misonidazole, a 2-nitroimidazole radiation sensitizer, was reduced in dilute aqueous solution by irradiation in the absence of oxygen and in the presence of excess formate ions. In solutions buffered at pH 7.0 the misonidazole absorption spectrum is lost with increasing radiation exposure, consuming 4.0 + 0.2 reducing equivalents per molecule, with no evidence for the formation of products absorbing in the range of 220 to 500 nm. HPLC separations of these samples produced two peaks, one containing 80-90% of the radioactivity, which grew at the same rate as misonidazole was lost. These peaks did not contain the amine, azo, azoxy, or hydrazo derivatives and had only very weak absorption (E254 = 100 M-1 cm-1). Although mass spectrometry of the main peak indicated the presence of some material with the m/e ratio expected for the hydroxylamine (187), repeated NMR analysis demonstrated that the yield of this derivative constitutes much less than 10% of the separated products. Irradiation at very high dose rate gave these same peaks and, in addition, a small yield of the amine derivative. Although reduction of misonidazole by the one-electron radical species produced by radiation occurs with four-electron stoichiometry, under these conditions the expected hydroxylamine derivative is unstable and cannot be recovered in large yield from the reduction mixture.