Orbital signatures of the methyl group in thymine are identified using information from both coordinate and momentum spaces, in comparison with RNA base uracil. The B3LYP/aug-cc-pVTZ//B3LYP/TZVP calculations show that the orbitals of methyl group may be identified as 9a′, 15a′, 2a″, and 25a′, respectively. Generally, large changes in orbital energies directly lead to large changes in orbital momentum distributions and orbital wavefunctions despite strong pyrimidine ring buffer (exceptions of 19a′ and 21a′ of thymine). A general conclusion about the chemical bindings of pyrimidine, cytosine, thymine, and uracil is obtained for the first time. © 2013 Wiley Periodicals, Inc.