A 1:1 hydrogen-bonded complex between 2-pyridone and formic acid has been characterized using laser-induced-fluorescence excitation and dispersed fluorescence spectroscopy in a supersonic jet expansion. Under the same expansion condition, the fluorescence signal of the tautomeric form of the complex (2-hydroxypyridine(...) formic acid) is absent, although both the bare tautomeric molecules exhibit well-resolved laser-induced-fluorescence spectra. Quantum chemistry calculation at the DFT/B3LYP/6-311++G** level predicts that in the ground electronic state the activation barrier for tautomerization from hydroxy to keto form in bare molecules is very large ( similar to 34 kcal/ mol). However, the process turns out to be nearly barrierless when assisted by formic acid, and double proton transfer occurs via a concerted mechanism.