The electronic structures of pyrimidine and its substituted derivs. were studied by UPS and quantum chem. methods. The ionization potentials corresponding to the p MOs p1-p3 and the two nN orbitals of the pyrimidine unit could be detd. and assigned for all compds. Multiple linear regression analyses of the IPs related to these orbitals with different substituent consts. indicated that Hammett sP values performed well for all these IPs, whereas the resonance parameters R and R+ were satisfactory for p and poor for nN ionizations.