By means of the direct quantitative determination of the quantum yields of all deactivation channels of the first excited singlet state of selected alkyl substituted phenols it is shown, that sterically hindered (2,6-di-tert-butyl substituted) phenols (SHP) have extremely short singlet lifetimes and relax dominantly by radiationless processes. A quantum-chemical molecular analysis (vibration analysis, steric factors) gives evidence for an intense coupling interaction of the phenol group with the bulky substituents. (C) 2002 Elsevier Science B.V. All rights reserved.