The mechanism of gas-phase aromatic substitution by (CH3ONO2H+ ions has been studied by a combination of FT-ICR mass spectrometry and atmospheric pressure radiolytic techniques. Clarifying a long-standing ambiguity, the ICR results characterize the CH3OH-NO2+ complex (1), in essence a nitronium ion solvated by a methanol molecule, as the nitrating agent, whereas the CH3NO2H+ isomer (2) is devoid of nitrating properties and reacts with benzene exclusively as a Bronsted acid. Indeed, the reaction with benzene has been exploited as an ICR "titration" technique to evaluate the relative abundances of 1 and in mixed populations of (CH3ONO2H+ ions from different preparative procedures. Radiolytic nitration of p-H-toluene-d(7) and p-D-toluene-h(7) leads to intraannular hydron migration from the ipso nitrated position, whose rate has been estimated to be ca. 1.6 x 10(6) s(-1) at 315 K. The mutually supporting evidence from:the ICR and the radiolytic experiments outlines a reaction mechanism involving preliminary formation of a Wheland intermediate from the attack of 1 on the arene, followed by its isomerization into the more stable O-protonated nitrobenzene structure via a proton shift whose rate is estimated to be ca. 3.6 x 10(7) s(-1) at 315 K. The results are compared with those of a recent theoretical analysis of the mechanism of aromatic nitration by isomeric (CH3ONO2H+ ions, and their correlation with condensed-phase nitration is briefly discussed.