The unimolecular reactivity of both protonated forms of ethyl cyanide (1) and ethyl isocyanide (2) in the gas phase has been examined experimentally using deuterium labeling and mass analyzed ion kinetic energy spectrometry. It has been shown that both species isomerize into weak pi-complexes between ethene and HCNH+ (3 and 4), which may further dissociate or interconvert either by reorientation or by cycloaddition-cycloreversion. Relevant portions of the C3H6N+ potential energy surface have been explored using ab initio molecular orbital calculations up to the MP2/6-311++G**//MP2/6-31G** + ZPE level. These data provide new examples of the 1,2 elimination processes which require the intermediacy of loosely bound ion-neutral complexes. This study is also of interest in interstellar and planetary atmosphere chemistry, particularly in view of the possible formation of ethyl isocyanide along with ethyl cyanide by dissociative recombination of both protonated forms 2 or 1, respectively.