Ab initio SCF and SCF-CI calculations have been performed to investigate substituent effects on ground-and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2-and 4-R-pyridines, with R including the 7r donating and u withdrawing groups CH3, NH2, OH, F, and C2H3 and the 0 and w electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a w donating group leads to an increase in the n+x* transition energy of 4-R-pyrimidines, while the w withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy difference:; and virtual excitation energies tend to correlate with n -v * transition energies of 4-Rpyrimidines with saturated R groups, but such correlations are masked by w conjugation, n orbital interaction, and configurational mixing when the unsaturated groups CzH3, CHO, and CN are present. The electronic effects of a 7r donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a w withdrawing group are weaker when the group is bonded to pyrimidine