Bactcriorhodopsin is the light-transducing protein in the purple membrane of Halobacterium halobium. Irradiation of the light-adapted form (bR) initiates a photocycle that pumps protons across the membrane. An accurate assignment of the energy storage associated with the primary event of bR is important to an understanding of the molecular mechanism and the stoichiometry of proton pumping. Previous photocalorimetric studies have concluded that ~16 kcal mol-1 is stored in the K photoproduct, an energy sufficient to pump two protons per photocycle.