Abstract The potential energy profiles for the fragmentations that lead to [C 5 H 5 O] + and [C 4 H 6 ] +• ions from the molecular ions [C 5 H 6 O] +• of E ‐2,4‐pentadienal were obtained from calculations at the UB3LYP/6‐311G + + (3df,3pd)//UB3LYP/6‐31G(d,p) level of theory. Kinetic barriers and harmonic frequencies obtained by the density functional method were then employed in Rice–Ramsperger–Kassel–Marcus calculations of individual rate coefficients for a large number of reaction steps. The pre‐equilibrium and rate‐controlling step approximations were applied to different regions of the complex potential energy surface, allowing the overall rate of decomposition to be calculated and discriminated between three rival pathways: CH bond cleavage, decarbonylation and cyclization. These processes should have to compete for an equilibrated mixture of four conformers of the E ‐2,4‐pentadienal ions. The direct dissociation, however, can only become important in the high‐energy regime. In contrast, loss of CO and cyclization are observable processes in the metastable kinetic window. The former involves a slow 1,2‐hydrogen shift from the carbonyl group that is immediately followed by the formation of an ion‐neutral complex which, in turn, decomposes rapidly to the s ‐ trans ‐1,3‐butadiene ion [C 4 H 6 ] +• . The predominating metastable channel is the second one, that is, a multi‐step ring closure which starts with a rate‐limiting cis — trans isomerization. This process yields a mixture of interconverting pyran ions that dissociates to the pyrylium ions [C 5 H 5 O] + . These results can be used to rationalize the CID mass spectrum of E ‐2,4‐pentadienal in a low‐energy regime. Copyright © 2010 John Wiley & Sons, Ltd.