AbstractUnimolecular dissociation of the molecular cations of 1,2‐expoxybutane (1), 1‐methoxypropene (2), 2,3‐epoxybutane (3), ethyl‐vinyl ether (4), 1,2‐epoxy‐2‐methylpropene (5), and 2‐methoxypropene (6)have been investigated. Different behaviour of pairs of [C4H8O]+˙ isomers (1+˙/2+˙), (3+˙/4+˙) and (5+˙/6+˙) have been demonstrated. Isomerization by epoxide ring opening and 1,4‐hydrogen migration is suggested to explain the similar unimolecular chemistry observed for the two partners