Fragmentation behavior upon low energy (10-16 eV) electron molecule collision of two isomeric alpha,beta-enones, crotonaldehyde and methacrolein, has been studied by quadrupole mass spectrometry. Three predominant reaction channels identified immediately above the ionization threshold are H, CO and HCO losses from the parent molecular ions, and isomeric effects are vividly manifested with respect to these channels. Signals corresponding to CH3C=O+ ion, produced due to methyl migration, are displayed only by methacrolein. The observed isomeric effects are interpreted using the energies of the transition states and thermodynamic stabilities of the products predicted by calculation at DFT/B3LYP/6311++G(d,p) level. (C) 2013 Elsevier B. V. All rights reserved.