From the results of polarized absorption spectra and MO calculations, the electronic structures of p -diethylamino and p -nitro substituted benzenediazonium cations (The unsubstituted cation is abbreviated to BAF, and the p -diethylamino and p -nitro substituted cations to p -BAF-N (C₂H₅) ₂ and p -BAF-NO₂, respectively.) have been discussed. The first and second excited states wavefunctions (Ψ⁰₁ and Ψ⁰₂) of BAF belong to ¹ B ₂ and ¹A₁ ( C_2υ pdint group), respectively. The first excited state wavefunction (Ψ₁) of p -BAF-NO₂ is a type of ¹B₂ symmetry and corresponds to Ψ⁰₁ of BAF, while Ψ₁, of p -BAF-N (C₂H₅) ₂ is of ¹A₁ symmetry and corresponds to Ψ⁰₂ of BAF, i, e., the origin of the first singlet excited state of p -BAF-NO₂ is quite different from that of p -BAF-N (C₂H₅) ₂. The differences in the photochemical reactivities in both compounds may be attributable to the differences in the nature of the first excited states.