The pH effects on photochemical reactions of guanine (Gua), guanosine (Guo) and 2'-deoxyguanosine-5'-monophosphoric acid (dGMP) using acetone as a photosensitizer have been investigated by 248 nm laser flash photolysis. Laser-induced transient species have been characterized according to kinetic analysis and quenching experiments by Mn 2 + and 02. In acid aqueous solutions, the intermediates recorded are assigned to the excited triplet states and the dehydrogenated radicals of Gua, Guo and dGMP. The excited triplet states produced via triplet-triplet excitation transfer and the dehydrogenated radicals arising from electron transfer reactions have been identified for the first time. In alkaline aqueous solutions, transient absorption spectra of dehydrogenated radical anions have been observed, but no triplet states of Gua, Guo and dGMP could be detected, The kinetics of the formation and decay of the exciplex arising from interaction of 3A* and Gua [ -H ~ ]and its derivatives have been offered for the first time. However, in neutral aqueous solution the intermediates recorded are assigned to the excited triplet state and dehydrogenated radical of dGMP arising from energy transfer and an electron transfer reaction. The sites of dehydrogenation have been suggested. Furthermore, four sets of kinetic parameters of the triplet decay of Gua and its derivatives have also been determined.