The physicochemical processes underlying the radiation sensitization of DNA by the K-shell photoabsorption of nitrogen and oxygen atoms are unclear. Accordingly, we evaluated electron paramagnetic resonance spectra of thin films of pyrimidine bases. By altering the substituents of the pyrimidine ring, we found a substituent effect on the yield of unpaired electrons induced by photoabsorption. The yield of the unpaired electrons of the DNA-base containing amino groups increased by the donation of electrons, and the yield of halogenated DNA-bases decreased by the withdrawal of electrons. These different physicochemical processes might affect DNA bases, leading to differences in DNA modification.