AbstractThe energetic lowest excited singlet and triplet states, the quantum yields of fluorescence, intersystem crossing, and internal conversion, the singlet life times, the fluorescence anisotropy and the dipol moments of the ground and the excited states of benzo[g]quinoline are determined. The dependence of the fluorescence quantum yield on temperature is attributed to a thermal activated deactivation process of the first excited singlet state by intersystem crossing. Considering a contribution of temperature independent radiationless deactivation this temperature dependence obeys the Arrhenius equation. ‐ At increasing acceptor behaviour and ET‐values of the solvent the activation energy of temperature dependent intersystem crossing is increasing too. A linear relationship between the experimental determined activation energies and the solvent parameter (ET+ b ·NA), (NA= Acceptor number,b= 8.3 J · mol−1), is obtained. The rate constants of the temperature dependent intersystem crossing process are different in the cases of investigated solvents and correlate with the above‐mentioned solvent parameter, provided heavy atomes are absent. These results are explained by the different solvation of S1(π π*‐character) and T3(π π*‐with partial nπ*‐character). — At 77 K no phosphorescence can be observed (øp≤ 0.01). It follows that the temperature independent intersystem crossing process is negligible. — The quantum yields of internal conversion decrease at increasing solvent parameter. This fact is attributed to the weaker coupling of the S1(π π*) and S2(n π*) states in the case of increasing energy distance and, connected with this, smaller Franck‐Condon‐