Photoinduced electron transfer between water-oil phases is accomplished in a water-in-oil microemulsion system using a "shuttle photosensitizer" as an electron transporter. The system consists of a water-in-oil microemulsion in which ethyl eosin, (1)EtEo(-), acts as a photosensitizer, Fe(CN)(6)(3-) as an electron acceptor, and tributylamine, Bu3N, as an electron donor. The hydrophilic photosensitizer and electron acceptor are solubilized in the aqueous microdroplets of the water-in-oil microemulsion, whereas the hydrophobic electron donor is present in the continuous oil phase. Photoinduced oxidative electron-transfer quenching in the water phase, k(q) = 1.7 x 10(5) s(-1), results in the oxidized photosensitizer, (2)EtEo(.), and Fe(CN)(6)(4-). The hydrophobic oxidized photosensitizer is extracted to the continuous oil phase, resulting in the phase separation of the photogenerated redox species, and the stabilization of the products against back electron transfer, k(rec) = 7.1 x 10(2) s(-1). The stabilization of the redox species against back electron transfer enables the reduction of the oxidized photosensitizer, (2)EtEo(.).by Bu3N in the oil phase, k(red) = 1.1 x 10(6) M-1.s(-1). The latter process regenerates the hydrophilic photosensitizer, (1)EoEt(-), that is transported back to the water microdroplets, a process leading to the electron transfer across the water-oil boundary by the shuttle photosensitizer. The photosensitized reduction of Fe(CN)63- by Bu3N, in the water-in-oil microemulsion system, proceeds with a quantum yield of phi = 0.04. The mechanism involved in the photoinduced electron transfer between the water-oil phases is elucidated by time-resolved laser flash photolysis experiments and steady-state irradiation. A detailed mathematical model, assuming a Poisson distribution of the quencher in the water droplets, is formulated. This accounts for the different processes involved in the electron transfer in the microheterogeneous system.