Time-resolved thermal lensing (TRTL) technique was applied to the determination of enthalpy change in double proton transfer reaction of 7-azaindole (AI) dimer and its complexes with acetic acid (AcOH) and methanol in solution. Excited AI dimer/complexes emit violet fluorescence or undergo rapid double proton transfer (DPT) reaction to form excited tautomers, which emit green fluorescence. The emission quantum yields were successfully determined with a calorimetric standard of pyridazine to be 0.00(6) (Al dimer), 0.16 (Al dimeric tautomer), 0.00(2) (AI-AcOH complex), 0.11 (AI-AcOH tautomer), and 0.08 (AI-methanol complex). By analyzing the time evolution of TRTL signals of AI dimer, the lifetime of the tautomer in the ground state was determined to be 19 mu s, which well agreed with the values obtained with flash photolysis and stepwise two-color laser-induced fluorescence experiments. The enthalpy change of the tautomer to the AI dimer in the ground state was obtained to be surprisingly low, 340 cm(-1). The TRTL results under high photon density excitation show that AT dimer easily absorbs second photon to dissociate into two monomeric tautomers followed by the recombination in ca. 2 ms.