The mechanism of nonradiative decay of singlet excited tryptophan (Trp) in aqueous solution was investigated by a highly selective photosubstitution of the C-4 hydrogen of Trp with deuterium of solvent D/sub 2/O. It was concluded that intramolecular proton transfer from the ..cap alpha..-ammonia group giving rise to formation of a protonated species plays an important role in the nonradiative decay