The effect of water on the primary photosynthetic activity of purple bacterium Rhodospirillum rubrum was studied in Hexadecane-Tween-Spane (HTS)- and phospholipid (PLC)-reverse micelles. Reverse micelles offer the possibility of modulating the amount of water to which enzymes and multienzymatic complexes are exposed. Fast bacteriochlorophyll (BChl) fluorescence induction kinetics and reaction centre absorption changes at 820 nm were used as an assay for the functional transfer of bacterial cells into HTS-reverse micelles and bacterial photosynthetic complexes (BPC) into PLC-reverse micelles. Both the bacterial cells and BPC showed an increase in the rate of primary photosynthetic activity by increasing the concentration of water in the reverse micelles. The bacterial cells could be kept viable for many hours in HTS-reverse micelles in presence of 6% (v/v) water. NMR studies indicated that the photosynthetic activity was affected by the availability of water in reverse micelles. The bacterial cells in HTS or BPC in PLC reverse micelles could be used to further understand the influence of water on the organisation and function of photosynthetic complexes. and A. Srivastava, A. Darszon, R. J. Strasser.