A recently developed method for time-resolved thermodynamic measurements was used to study the photochemical reaction(s) of the BLUF domain of AppA (AppA-BLUF), which has a dimeric form in the ground state, in terms of the energetics and heat capacity changes (Delta C-p) in different time domains. The enthalpy change (Delta H) of the first intermediate that forms within 1 ns after photoexcitation was 38 (+/-8) kJ mol(-1) at 298 K. The heat capacity change (Delta C-p) upon formation of this intermediate was positive [1.4 (+/- 0.3) kJ mol(-1) K-1]. This positive Delta C-p suggests that the hydrophobic surface area of AppA-BLUF exposed to the bulk solvent increased. After this initial transition, a dimerization reaction with another ground-state dimer (i.e., tetramer formation) takes place. Upon this reaction, the energy was stabilized to 26 (+/- 6) kJ mol(-1) at 298 K. Interestingly, the dimer formation was accompanied by a larger but negative Delta C-p [-6.0 (+/- 1) kJ mol(-1) K-1]. This negative Delta C-p might indicate buried hydrophobic residues at the interface of the dimer and/or the existence of trapped water at the interface. We suggest that hydrophobic interactions are the main driving force for the formation of the dimer upon photoactivation of AppA-BLUF.