The 2,6- and 3,5-dimethylenepyridinium biradical ions were generated and structurally characterized in the gas phase by using Fourier transform ion cyclotron resonance mass spectrometry. Their reactivity was examined toward several commonly used spin traps. Reaction rates and product distributions were determined. The isomeric biradicals were found to possess remarkably different chemical properties. Most importantly, the 2,6-dimethylenepyridinium biradical is highly reactive toward all the reagents studied, while the 3,5-dimethylenepyridinium biradical reacts slowly or not at all. The reaction of the 2,6-dimethylenepyridinium biradical with tert-butyl isocyanide occurs via HCN abstraction and addition, the same pathways observed for isomeric closed-shell pyridylmethyl cations. In contrast, the 3,5-dimethylenepyridinium biradical reacts exclusively by slow addition. The same reaction was observed for the analogous 3-methylenepyridinium monoradical ion. The 3,5-dimethylenepyridinium biradical and the 3-methylenepyridinium monoradical also display similar reactivity toward the other reagents studied, while this is not the case for the 2,6-dimethylenepyridinium biradical and the pyridylmethyl cations. Some of the differences observed in the reactivity of the isomeric biradicals are likely explained by their different ground-state multiplicities. Earlier ab initio molecular orbital calculations predict a triplet ground state for the 3,5-dimethylenepyridinium biradical (by about 6 kcal/mol) and a closed-shell singlet ground state for the 2,6-dimethylenepyridinium biradical (by about 2 kcal/mol). The different location of the protonated nitrogen relative to the methylene groups probably has a minor influence over the types of reaction products obtained since this is the case for the analogous monoradicals.