The efficiency of photodynamic reactions depends on 1), the penetration depth of the photosensitizer into the membrane and 2), the sidedness of the target. Molecules which are susceptible to singlet oxygen ((1)O(2)) experience less damage when separated from the photosensitizer by the membrane. Since (1)O(2) lifetime in the membrane environment is orders of magnitude longer than the time required for nonexcited oxygen (O(2)) to cross the membrane, this observation suggests that differences between the permeabilities or membrane partition of (1)O(2) and O(2) exist. We investigated this hypothesis by releasing (1)O(2) at one side of a planar membrane while monitoring the kinetics of target damage at the opposite side of the same membrane. Damage to the target, represented by dipole-modifying molecules (phloretin or phlorizin), was indicated by changes in the interleaflet dipole potential difference Deltaphi(b). A simple analytical model allowed estimation of the (1)O(2) interleaflet concentration difference from the rate at which Deltaphi(b) changed. It confirmed that the lower limit of (1)O(2) permeability is approximately 2 cm/s; i.e., it roughly matches O(2) permeability as predicted by Overton's rule. Consequently, the membrane cannot act as a barrier to (1)O(2) diffusion. Differences in the reaction rates at the cytoplasmic and extracellular membrane leaflets may be attributed only to (1)O(2) quenchers inside the membrane.