Abstract This report describes a new method for detection and identification of intermediate radical species in the gas phase. It provides mass spectra of quasi-molecular ions formed by lithium-ion attachment to the radical species (R) under high pressure. Results are obtained in the form of a mass spectrometric trace of Li + adduct radicals. As an example, the method was applied to the study of the microwave discharge in i-butane/He and was successfully used for the qualitative analysis of the compositions of hydrocarbon radicals in the discharge. Various kinds of hydrocarbon radicals, such as C n H 2 n−1 and C n H 2 n+1 (where n=2–6), were identified.