The mechanism of action of photosensitizers used in the photodynamic tumor therapy is studied by using bilayer lipid membranes (BLM) which adequately simulate the plasma membranes of cells. The formation of singlet oxygen upon photoexcitation of photosensitizer (tetrasulfonated alumophthalocyanine) is revealed by the photosensitizer-induced decomposition of styryl dyes di-4-ANEPPS or RH-421 the adsorption of which on BLM gives rise to changes in the dipole potential at the membrane interface. This potential decreases upon illumination of the membrane covered with adsorbed phthalocyanine and dye molecules as a result of the dye oxidation by singlet oxygen and is recovered upon interruption of illumination as a result of adsorption of new dye molecules from the solution. The changes in the potential are observed both when the dye molecules are adsorbed on the same (cis) membrane side as the photosensitizer molecules and also when they are adsorbed on the opposite (trans) membrane side. When the dye molecules are adsorbed on the trans side, the kinetics of potential variations in the light and its subsequent recovery in the dark depends on the membrane size. This dependence is explained within the framework of a model which assumes that dye molecules can get into the lipid bilayer not only from the aqueous solution but also as a result of lateral diffusion from the bilayer-surrounding circular reservoir containing a lipid solution in decane. The quantitative description of experimental data in terms of this model allows the coefficient of lateral diffusion of di-4- ANEPPS molecules in the lipid bilayer to be assessed as 10–7–10–6 cm2/s. The oxidation rate of dye molecules on the cis side is shown to be lower, which is explained by inhibition of the formation of singlet oxygen by dye molecules.