The dynamics of the amino-imino double proton-transfer tautomerism reaction of the 2-aminopyridine (2AP)/acetic acid system in hexane have been investigated with steady-state absorption, steady-state fluorescence, and picosecond time-resolved fluorescence spectroscopies. It has been confirmed that the double proton-transfer reaction takes place in the excited state of the double hydrogen-bonded complex of 2AP with acetic acid. The imino tautomer fluorescence shows a rise behavior with a 5 ps time constant at 480 nm, while a decay with a 5 ps time constant has been observed at 360 nm. The rate of this kinetics is reduced to 7 ps with deuterium substitution. From the comparison with the steady-state fluorescence spectra of proton-transferred model compounds, the 5 ps decaying species has been identified as the intermediate in which one proton is transferred from acetic acid to 2AP. It is thus concluded that the 5 ps time constant represents the second proton-transfer process which follows the first proton transfer that is too fast to be detected in the present experiment, The photoexcited double hydrogen-bonded complex of 2AP and acetic acid undergoes a stepwise double proton-transfer reaction within 5 ps in the excited state.