Substituent effects on the energies (E-ob) of electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations 5a-k(.+) and those of structurally related 1,1-diarylethyl cations 7a-k(+) were determined experimentally by using electronic transition spectroscopy. In addition, transition energies of these radical cations were determined by using density functional theory (DFT) and time-dependent (TD)-DFT calculations. The electronic transition bands of 5a-k(.+) and 7a-k(+) have maxima (lambda(ob)) that appear at 500-432 and 472-422 nm, respectively. A Hammett treatment made by plotting the E-ob values relative to that of the diphenyl-TMM radical cation 5d(.+) (Delta E-ob) vs the cationic substituent parameter sigma(+) give a favorable correlation with a boundary point at sigma(+) = 0.00 and a positive rho for sigma(+) < 0 and a negative rho for sigma(+) > 0. A comparison of the lambda(ob) and rho values for 5a-k(.+) and 7a-k(+) suggests that the chromophore of 5(.+) is substantially the same as that of 7(+). The results of TD-DFT calculations, which reproduce the experimental electronic transition spectra and relationships between Delta E-ob and sigma(+), and suggest that the absorption band of 5(.+) is associated with the SOMO-X -> SOMO transition, while that of 7(+) is due to the HOMO -> LUMO transition. Another interesting observation is that Cl and Br substituents in the diphenyl-substituted TMM radical cations and 1,1-diarylethyl cations 7a-k(+) act as electron-donating groups in terms of their effect on the corresponding electronic transitions. The results show that the molecular structure of 5(.+) is a considerably twisted and that 5(.+) has a substantially localized electronic state in which the positive charge and odd electron are localized in the respective diarylmethyl and the allyl moieties.