In order to obtain insight into the protonation of the electron adducts of pyrimidine bases (I) and the chemical behavior of the resulting radicals under an acidic condition, the photoreaction of 5-substituted 1,3-dimethyluracil (1) (substituents; p-xylyl, methyl, H, Cl, and F) was performed in p-xylene (2) in the presence of a large amount of trifluoroacetic acid (TFA). 5, 6-dihydrouracil derivatives (3), 5-p-methylbenzyluracils (4) and the 6-isomers (5) were yielded as the major products. Substitution of an electron-releasing group (p-xylyl) at C-5 raises the ratio of the products from the C6-protonated pyrimidine electron adduct (III) (3, 4) vs. the adduct from the O4-protonated isomer (II) (5), while the electron-withdrawing substituents (F > Cl) suppress the ratios.