The rate of efflux of trapped 5/6-carboxyfluorescein from sealed lipid vesicles showed a marked dependence on (a) temperature, (b) phospholipid acyl chain composition, and (c) the nature of co-trapped counterions. When the dye was salted with sodium, at pH greater than 7, the rate of dye permeation showed a discrete maximum at the melting point of the lipid bilayer (Tc); in the case of membranes composed of dipalmitoylphosphatidylcholine, this discontinuity extended over a very broad temperature range, being detectable at least 10 degrees C above and below Tc. The peak in dye permeation rate was superimposed on a permeation profile that showed a simple exponential relationship to temperature. Studies with a homologous series of saturated lecithin bilayers revealed a consistent pattern of behavior: a logarithmic dependence of dye permeation rate on temperature with a superimposed discontinuity at Tc. For thin membranes (12-14-carbon acyl chains), the discontinuity was severe, exerting an influence over a very broad temperature range and leading to extremely high overall dye leakage rates. As the acyl chains were lengthened, the discontinuity became less pronounced, almost disappearing at a chain length of 20 carbons. In sharp contrast to these results, dye salted with N-methylglucamine [or with tris(hydroxymethyl)aminomethane] showed no efflux maximum at Tc, and base-line leakage rates were generally slower. When dye was salted with ammonium, efflux was too rapid to monitor, even at temperatures well below Tc. The results indicate that the rate of release of electrically charged dyes, such as 5/6- carboxyfluorescein, from sealed lipid vesicles can be tightly coupled to the counterion leakage rate and hence can provide an accurate and convenient assay of relative ion flux across phospholipid bilayers.