A comparative analysis was performed for 13C NMR chemical shifts of C-4 and C-4a (or C-5) atoms in previously synthesized pyrido[2,3-d]-pyrimidin-5-ones, pyrido[2,3-d]pyrimidin-7-ones, pyrimido[4,5-d]pyrimidines, and 5-acetylpyrimidines containing methylsulfanyl, methylsulfonyl, butoxy, and amino groups at position 4. It was possible to determine the type of heteroatom (sulfur, oxygen, or nitrogen) bonded to the C-4 carbon atom on the basis of chemical shifts observed for C-4a carbon atom in the spectra of fused heterocycles or C-5 carbon atom in the spectra of pyrimidines. At the same time, the chemical shift values of C-4a (or C-5) atoms practically did not depend on the particular sulfur-containing functionality: sulfanyl or sulfonyl group. A significant upfield shift was observed for the C-4 atom in 13C NMR spectra of sulfones compared to the spectra of compounds containing a methylsulfanyl moiety.