Stopped flow measurements to determine the permeability of vesicles are presented, The kinetics of the reaction between FeSCN2+ and F- ions is used to monitor the permeability of vesicles, Samples with vesicles that have been equilibrated with the iron complex are mixed with F- solutions, The reaction is followed by UV/VIS absorption, The influence of temperature and surfactant concentration on the membrane permeability of large unilamellar phospholipid vesicles was studied, A dramatic increase of the permeability of the LUVs is observed when 30 to 40 mol% of the surfactant OP-10 (main component of Triton X-100) is added to the lipid, It is assumed that the increased permeability is due to the stabilization of transient defects in the bilayers of the vesicles as shown previously by other groups, Furthermore, a strong binding of the iron (III) thiocyanate complex to the phospholipid is observed by UV/VIS spectroscopy and zeta-potential measurements, Additional experiments with vesicles from a fluorocarbon surfactant show a much higher permeability than the phospholipid system, Models for the diffusion of either the iron (III) complex or the fluoride ions through the vesicles bilayer are discussed for LUV as well as for vesicles from a fluorocarbon surfactant, The results indicate that the rate-determining step is the diffusion of the iron complex through the membrane,