Liposomes,n-hexane phospholipids reverse micelles and organogel have been used as model membrane systems in which the photosynthetic reaction centres (RCs) fromRb. Sphaeroides have been incorporated. The influence of the liposomial composition on the RC photochemical properties have been tested, using liposomes made of different phospholipids. The charge recombination of flash generated states of the RC solubilized in reverse micelles have been followed as a function of the quinone concentration at two different temperatures. In this mimetic membrane system the quinone molecules are in fast exchange between theQ_B site of the protein and the bulk organic phase. The thermodynamic parameters relative to the electron transfer fromQ_A⁻ toQ_B and to the binding processes of the quinone to theQ_B site have been determined. Complex multiphasic kinetics for RCs in organogel have been observed. In this system, both the quinone binding to theQ_B site and theP⁺Q_A⁻ state stabilization are markedly influenced by theW₀. The role played by the water activity in the RC binding affinity for the ubiquinone molecule has been considered.