Cation radicals of guanine (G(center dot+)), 9-methylguanine (MG(center dot+)), and guanosine (rG(center dot+)) were generated by dissociative oxidation of gas-phase copper complexes and characterized by UV-vis photodissociation action spectra and ab initio calculations. Comparison of the action spectra of G(center dot+) with the calculated vibronic absorption spectra of several cation radical tautomers showed the best match for the canonical 6-oxo-N-9-H structure (G1(center dot+)). The formation of G1(center dot+) was favored by the stability of its precursor Cu-II ion complexes in solution and the gas phase. Gl(center dot+) was the marginally lowest-energy guanine tautomer according to CCSD(T) calculations extrapolated to the complete basis set limit (CBS). A canonical 6-oxo structure (MG1(center dot+)) was also assigned to the 9-methylguanine cation radical on the basis of a match between the action spectrum and the calculated vibronic absorption spectra. MG1(center dot+) was calculated by CCSD(T)/CBS to be marginally less stable than the 6-OH enol tautormer, but its formation was favored by the superior stability of its precursor Cu-II ion complexes in solution and the gas phase. Action spectroscopy allowed us to assign the canonical 6-oxo structure (rG1(center dot+)) to the gas-phase guanosine cation radicals that were formed as the lowest-energy tautomers. The absorption bands in the action spectra were assigned on the basis of time-dependent density functional theory calculations that were benchmarked on equation-of-motion coupled cluster calculations of G(center dot+).