Nitrite (O=N-O−, NO2−) and nitrate (O=N(O)-O−, NO3−) are ubiquitous in nature. In aerated aqueous solutions, nitrite is considered the major autoxidation product of nitric oxide (●NO).●NO is an environmental gas but is also endogenously produced from the amino acid L-arginine by the catalytic action of●NO synthases. It is considered that the autoxidation of●NO in aqueous solutions and in O2-containing gas phase proceeds via different neutral (e.g., O=N-O-N=O) and radical (e.g., ONOO●) intermediates. In aqueous buffers, endogenousS-nitrosothiols (thionitrites, RSNO) from thiols (RSH) such as L-cysteine (i.e.,S-nitroso-L-cysteine, CysSNO) and cysteine-containing peptides such as glutathione (GSH) (i.e.,S-nitrosoglutathione, GSNO) may be formed during the autoxidation of●NO in the presence of thiols and dioxygen (e.g., GSH + O=N-O-N=O → GSNO + O=N-O−+ H+; pKaHONO, 3.24). The reaction products of thionitrites in aerated aqueous solutions may be different from those of●NO. This work describes in vitro GC-MS studies on the reactions of unlabeled (14NO2−) and labeled nitrite (15NO2−) and RSNO (RS15NO, RS15N18O) performed in pH-neutral aqueous buffers of phosphate or tris(hydroxyethylamine) prepared in unlabeled (H216O) or labeled H2O (H218O). Unlabeled and stable-isotope-labeled nitrite and nitrate species were measured by gas chromatography–mass spectrometry (GC-MS) after derivatization with pentafluorobenzyl bromide and negative-ion chemical ionization. The study provides strong indication for the formation of O=N-O-N=O as an intermediate of●NO autoxidation in pH-neutral aqueous buffers. In high molar excess, HgCl2accelerates and increases RSNO hydrolysis to nitrite, thereby incorporating18O from H218O into the SNO group. In aqueous buffers prepared in H218O, synthetic peroxynitrite (ONOO−) decomposes to nitrite without18O incorporation, indicating water-independent decomposition of peroxynitrite to nitrite. Use of RS15NO and H218O in combination with GC-MS allows generation of definite results and elucidation of reaction mechanisms of oxidation of●NO and hydrolysis of RSNO.