Abstract The potential energy profiles for the mutual conversion of the isomeric molecular ions [C 5 H 6 O] +• of 2‐methylfuran, 3‐methylfuran and 4 H ‐pyran and the fragmentations that lead to [C 5 H 5 O] + ions were obtained from calculations at the B3LYP/6‐311G + + (3df,3pd)//B3LYP/6‐31G(d,p) level of theory. The various competing unimolecular processes were characterized by their RRKM microcanonical rate coefficients, k ( E ), using the sets of reactant and transition state frequencies and the kinetic barriers obtained from the density functional method. In either a high‐ or a low‐energy regime, the pyrylium ion [C 5 H 5 O] + is generated directly from the 4 H ‐pyran molecular ion by a simple cleavage. In contrast, in the metastable kinetic window, the molecular ions of methylfurans irreversibly isomerize to a mixture of interconverting structures before dissociation, which includes the 2 H ‐ and 3 H ‐pyran ions. The hydrogen atoms attached to saturated carbons of the pyran rings are very stabilizing at the position 2, but they are very labile at position 3 and can be shifted to adjacent positions. Once 4 H ‐pyran ion has been formed, the CH bond cleavage begins before any hydrogen shift occurs. According to our calculation, there would not be complete H scrambling preceding the dissociation of the molecular ions [C 5 H 6 O] +• . On the other hand, as the internal energy of the 2‐methylfuran molecular ion increases, H • loss can become more important. These results agree with the available experimental data. Copyright © 2009 John Wiley & Sons, Ltd.