Theoretical approaches on the ground- and excited-state proton (or hydrogen atom) transfer in the 3-formyl-7-azaindole (3FAI)/formic acid dual hydrogen-bonded complex were performed. In the ground state, the analysis of the transition-state geometry led us to conclude a concerted, asynchronous proton-transfer pattern that correlates with the hydrogen-bonding strength. The lowest singlet excited state in the 3FAI/formic acid complex was calculated to be in an n pi* configuration. On the basis of frontier molecular orbital analyses, the n --> pi* transition was concluded to originate from the carbonyl lone-pair electrons of the formyl substitute. A highly endergonic proton-transfer reaction barrier of similar to 16.7 kcal/mol was calculated in the (1)n pi* state at the CIS/6-31G(d',p') level of theory. The second excited singlet state possesses a pi pi* configuration in which the excited-state double proton transfer (ESDPT) takes place with a negligible energy barrier. The results provide a theoretical rationalization of the competitive internal conversion/ESDPT mechanism previously proposed for the 3FAI hydrogen-bonded complexes (J. Phys. Chem. A 2000, 104, 8863).