F-type H+-ATP synthases synthesize ATP from ADP and phosphate using the energy supplied by a transmembrane electrochemical potential difference of protons. Rotary subunit movements within the enzyme drive catalysis in either an ATP hydrolysis or an ATP synthesis direction respectively. To monitor these subunit movements and associated conformational changes in real time and with subnanometre resolution, a single-molecule FRET (fluorescence resonance energy transfer) approach has been developed using the double-labelled H+-ATP synthase from Escherichia coli. After reconstitution into a liposome, this enzyme was able to catalyse ATP synthesis when the membrane was energized.