The mechanism of the H-atom transfer from phenols or thiophenols to triplet pi,pi* 5,.12-naphthacenequinone (5,12-NQ) has been examined by means of laser flash photolysis at 295 K. Based oil the Hammett plots and the Rehm-Weller equation for the quenching rate constants, the phenolic H-atom transfer from phenols or thiophenols to triplet pi,pi* 5,12-NQ is shown to proceed via the electron transfer followed by proton transfer. The previously proposed mechanism for H-atom transfer of pi,pi* triplets, that proton transfer is followed by electron transfer, was not verified in the present systems. (c) 2006 Elsevier B.V. All rights reserved.