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Thin lipid (optically black) membranes were made from sheep red cell lipids dissolved in n-decane. The flux of Br across these membranes was measured by the use of tracer (82)Br. The unidirectional flux of Br (in 50-100 mM NaBr) was 1-3 x 10(-12) mole/cm(2)sec. This flux is more than 1000 times the flux predicted from the membrane electrical resistance (&gt;10(8) ohm-cm(2)) and the transference number for Br(-) (0.2-0.3), which was estimated from measurements of the zero current potential difference. The Br flux was not affected by changes in the potential difference imposed across the membrane (+/-60 mv) or by the ionic strength of the bathing solutions. However, the addition of a reducing agent, sodium thiosulfate (10(-3)M), to the NaBr solution bathing the membrane caused a 90% reduction in the Br flux. The inhibiting effect of S(2)O(3) (=) suggests that the Br flux is due chiefly to traces of Br(2) in NaBr solutions. As expected, the addition of Br(2) to the NaBr solutions greatly stimulated the Br flux. However, at constant Br(2) concentration, the Br flux was also stimulated by increasing the Br(-) concentration, in spite of the fact that the membrane was virtually impermeable to Br(-). Finally, the Br flux appeared to saturate at high Br(2) concentrations, and the saturation value was roughly proportional to the Br(-) concentration. These results can be explained by a model which assumes that Br crosses the membrane only as Br(2) but that rapid equilibration of Br between Br(2) and Br(-) occurs in the unstirred layers of aqueous solution bathing the two sides of the membrane. A consequence of the model is that Br(-) "facilitates" the diffusion of Br across the unstirred layers.

The Permeability of Thin Lipid Membranes to Bromide and Bromine