Thymine cation radicals were generated in the gas phase by collision-induced intramolecular electron transfer in [Cu(2,2':6,2 ''-terpyridine)(thymine)](2+center dot) complexes and characterized by ion-molecule reactions, UV-vis photodissociation action spectroscopy, and ab initio and density functional theory calculations. The experimental results indicated the formation of a tautomer ' mixture consisting chiefly (77%) of noncanonical tautomers with a C-7-H-2 group. The canonical 2,4-dioxo-N-1,N-3-H isomer was formed as a minor component at ca. 23%. Ab initio CCSD(T) calculations indicated that the canonical [thymine](+center dot) ion was not the lowest-energy isomer. This contrasts with neutral thymine, for which the canonical isomer is the lowest-energy structure. Exothermic unimolecular isomerization by a methyl hydrogen migration in the canonical [thymine](+center dot) ion required a low energy barrier, forming a C-7-H-2,O-4-H isomer. Noncanonical thymine tautomers with a C-7-H-2 group were also identified by calculations as low-energy isomers of 2'-deoxythymidine phosphate cation radicals. The relative energies of thymidine ion isomers were sensitive to the computational method used and were affected by solvation. The noncanonical [thymine](+)(center dot )ions have extremely low adiabatic recombination energies (REadiab < 5.9 eV), making them potential ionization hole traps in ionized nucleic acids.