Abstract—The energies of the lowest excited singlet, Es, and triplet, Et, states, and singlet‐triplet splitting energies, ΔEs,t, were determined on 18 carcinogenic and 31 noncarcinogenic polycyclic aromatics. A highly significant correlation was found between carcinogenic activity and the energy of the excited singlet state. Compounds with anEs<312 kJ/mol were 4.8 times more likely to be carcinogens than those compounds withEs312 kJ/mol (P= 0.015). Compounds whose singlet energies fell within the narrow range of 297 ≤Es≤ 310 kJ/mol were 22.8 times more likely to be carcinogens than those compounds which fell outside this range (P= 0.00006). A significant correlation between carcinogenic activity andEtenergies was not found, while the correlation involving ΔEs,tenergies was intermediate between theEsandEtcorrelations. The phosphorescence lifetimes, τp, of the 18 carcinogenic aromatics and 27 of the noncarcinogenic aromatip were determined, and were shown not to be correlated with carcinogenic activity. When either theEtor ΔEs,tenergies were plotted as a function ofEsit was found that the carcinogens tended to form in an elliptical cluster. Compounds whoseEsandEtenergies placed them within the ellipse were 9.7 times more likely to be carcinogens than those compounds which fell outside the ellipse (P= 0.002), while with theEs, ΔEs,tellipse, compounds which fell inside were 20.6 times more likely to be carcinogens than those which fell outside (P= 0.0004).Es, Et, ΔEs,tand τpvalues were also determined on 12 carcinogenic and 4 noncarcinogenic alkyl substituted benz[a]anthracenes. There was no significant difference between the carcinogens and noncarcinogens and the “elliptical” correlation predicted both the carcinogens and noncarcinogens to be carcinogenic. The results suggest that either some property(ies) of the lowest excited singlet state, but not its energy, or some molecular property(ies) which runs parallel to singlet state energies may be important in determining carcinogenic activity in po