The acid-catalyzed excited-state double-proton transfer (ESDPT) in 7-azaindole has been studied. In carboxylic acids and phosphoric acids, the formation of a 1:1 cyclic hydrogen bonded acid/7-azaindole complex was observed with a remarkably large association constant of >1.0 x 10(4) M(-1). In contrast to alcohol-catalyzed ESDPT, in which the slow ESDPT dynamics involve a large amplitude of solvent reorganization, the rate of acid catalyzed ESDPT is much greater than the decay rate (similar to 1.0 X 10(9) s(-1)) of the normal emission, resulting in a unique tautomer emission. The highly efficient acid-catalyzed ESDPT in 7-azaindole points to the biological application of 7-azaindole as a suitable acid derivative probe in a hydrophobic environment such as in the cell membrane.