We have used a capacitor microphone calorimeter to measure rapid enthalpy changes that occur when bacteriorhodopsin-containing membrane fragments are excited with short flashes of light. We resolved the enthalpy changes into three phases. At about 100 microsecond after the flash, the bacteriorhodopsins converted into metastable states have an enthalpy about 15–20 kcal mol-1 greater than the enthalpy before excitation. Some of this energy (approximately 10 kcal) is then released to the surroundings as the membrane fragments release protons to the solution. After proton release and before proton rebinding, a large amount of heat is released to the surroundings, equivalent to about 40–45 kcal/mol of bacteriorhodopsin reacting. At this point the energy of the system is about 35 kcal/mol less than it was before the flash; i.e., the system has released all of the energy of the photon (49 kcal/E) plus an additional 35 kcal/mol. Nevertheless, the free energy of the system must still be greater than it was originally, because relaxation to the original state occurs spontaneously. An entropy decrease of at least 125 cal/mol per deg is required to compensate for the heat release. An entropy decrease of this magnitude implies a major increase in molecular order in the purple membrane.