Ion-molecule reactions between ionized carbonyl compounds or ionized enols with t-butyl nitrite allow a clear-cut distinction to be made between both these isomeric structures. The most relevant difference between the observed reactions is the formal substitution of a hydrogen atom by nitric oxide in the enol ions. Collisional activation experiments on the product ions are interpreted in terms of alpha-nitroso carbonyl structures. However, a quantum chemical investigation at the B3LYP/6-311++G(d,p) level of theory shows that ionized alpha-nitroso carbonyl and vinyl nitrite structures may isomerize provided they contain ca 60 kj mol(-1) of internal energy. It is, therefore, possible that both structures are generated in the substitution reaction.