2-Methyl-2-phenyl-3-oxoindolin-1-yloxyl and 2-methyl-2-phenyl-3-aryliminoindolin-1-yloxyl radicals react with nitric oxide in the absence and presence of oxygen to form both substituted and unsubstituted N-nitro and N-nitroso stable compounds as the main products, while mono and dinitro compounds are formed in minor yields. In the presence of oxygen, the formation of the corresponding quinone imine N-oxide is observed. Mechanisms for the formation of the reaction products are proposed and discussed, leading to new insights into the chemistry of nitric oxide with aminoxyls. Crystal structures of 2-methyl-2-phenyl-N-nitrosoindolin-3-one and 2-methyl-2-phenyl-N-nitroindolin-3-one have been determined.