The rearrangement of the molecular ions of the isomeric 2-and fmethyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansionlring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1+'-4+' fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k,/k,, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the hehaviour of alkyl benzene ions, a unidirectional isomerization of the methyl henzofuran ions by ring expansion takes place hut no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.