Abstract18O‐multilabeled anhydrous nitric acid (89 atom %18O) was prepared by exchange with H218O, neutralization with NH3, and distillation from methane sulfonic acid. The use of trifluoromethane sulfonic acid, sulfuric acid, and oleum was also investigated, but these acids were found unsuitable due to rapid oxygen exchange. The extent of exchange was. monitored by converting water to methanol, nitric acid to nitroanisole, and analyzing by field ionization mass spectrometr