The cyanate anion (OCN)(-) may be formed in the ion source of a reverse sector VG ZAB 2HF mass spectrometer by dissociative electron capture of phenyl cyanate (PhOCN). The isomeric fulminate anion (ONC)(-) is formed by (i) deprotonation of nitromethane (with HO-) followed by elimination of H2O (-CH2NO2 --> [(CHN(O)(OH)](-) --> (ONC)(-) + H2O) and (ii) the reaction between CH2=N-OCH3 and O.- [CH2=N-OCH3 + O.- --> (ONC)(-) + H2O + CH3.]. Neutralization of the cyanate and fulminate anions by Franck-Condon vertical one-electron oxidation (in the first of two tandem collision cells of the mass spectrometer) yields the cyanate (OCN) and fulminate (ONC) radicals, both of which are stable for at least 1 mus. Ionization of these neutrals by one-electron oxidation in a second collision cell produces the cyanate and fulminate cations. A proportion of each of these cations undergoes isomerization. Theoretical and experimental studies show that (i) these rearrangements involve the less stable singlet forms of the cations, (ii) the cationic fulminate to cyanate rearrangement is a major process, and (iii) the corresponding rearrangement of the cyanate cation is a minor process, with the product being either the oxazirinyl cation or the fulminate cation.