It has long been assumed that ionizing neutral acetonitrile produces ions with the same atomic connectivity, CH3CN+center dot. Recent calculations on the C2H3N+center dot potential energy surface have suggested that it may be difficult to generate pure CH3CN+center dot when ionizing acetonitrile. We have probed the interconversion of CH3CN+center dot and its lower energy isomer CH2CNH+center dot by calculation, collision-induced dissociation mass spectrometry and ion-molecule reaction. The latter ion, ionized ketenimine, is co-generated upon electron or chemical ionization of neutral acetonitrile in the ion source of a mass spectrometer. An estimate of the ratio of the two isomers can be obtained from their respective ion-molecule reactions with CO2 or COS. CH3CN+center dot reacts by proton-transfer with CO2 and charge transfer with COS, whereas CH2CNH+center dot is unreactive.