In an electron impact dissociative ionization experiment on C 2H5OH, the formation of molecular ions requiring rearrangement of H atoms has been studied using a momentum spectrometer. H 3+, H2+, HOH+, and H 2OH+ observed in the experiment are molecular ions of this type. By comparing the mass spectrum of C2H5OH with that of its isotopomer C2H5OD, we determine the proportions of H-bond rearrangements involving carbon and oxygen sites. We find that the formation of H3+ due to the breaking of the O-H bond and rearrangement of the H atoms on the CH2 site is about 2.5 times as likely as its formation involving atoms from the CH3 site alone. No such difference is seen in case of the H2+ ion. The role of the O-H bond in formation of all observed ions has been assessed. Kinetic energy distributions of the molecular ions suggest that two or three electronically excited states contribute to their formation. © 2006 American Institute of Physics.