We report that (TMP)Ru(NH3)(2) (TMP = tetramesitylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri-tent-butylphenoxyl (ArO center dot), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH3)(2), leading to the discovery of a new catalytic C-N coupling to the para position of ArO center dot to form 4-amino-2,4,6-tri-tert-butylcyclohexa2,5-dien-1-one. Modification of the aryloxy radical to 2,6-di-tert-butyl-4-tritylphenoxyl radical, which contains a trityl group at the para position, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH3 to give N-2. We achieved 125 +/- 5 turnovers at 22 degrees C for oxidation of NH3, the highest turnover number (TON) reported to date for a molecular catalyst.