A new experimental technique is presented in which Fourier transform ion cyclotron resonance spectra may be used to examine the metastable dissociations of chemically activated adduct ions undergoing unimolecular dissociation on time scales greater than 50 mu s. The technique involves taking the difference between two spectra, one in which continuous rf irradiation at the cyclotron frequency of the transient adduct ion is carried out during a reaction delay and another in which no such irradiation occurs during the same reaction delay. The difference spectrum then yields a mass spectrum of all those adduct ions with metastable lifetimes greater than the time required to effect the ejection. The technique is applied to examples in mechanistic ion chemistry where the product distribution arising from unimolecular dissociation of the metastable ion is examined as a function of the lifetime probed, and this information is then used to deduce a probable reaction mechanism. The potential of the technique for direct determination of metastable ion lifetimes is noted.