A new reaction, the inorganic pyrophosphate-driven synthesis of ATP in Rhodospirillum rubrum chromatophores, was demonstrated. The effect of inhibitors such as m-Cl-CCP, S-13, desaspidin, and oligomycin indicate that this is an energy-linked reaction. Phosphotransferase and exchange reactions involving the direct transfer of phosphate from PP i to ADP or ATP were very low or nonexistent in chromatophores. Thus, the reaction can be described by the following equations where “~ X” represents an energized state or intermediate: PP i ⇌ “~ X” + 2 P i ADP + P i + “~ X” ⇌ ATP The characteristics and requirements for the reaction were elucidated and the interaction between this reaction and other energy-linked reactions, such as NAD + reduction, transhydrogenation, and ATP-P i exchange, was demonstrated. The discussion focuses on the advantage of this energy-conserving reaction to the cell.