Dissociation of fatty acids (FA) from and transbilayer movement (flip-flop) in small unilamellar phosphatidylcholine vesicles (SUV) were monitored by measuring the pH inside the vesicle with an entrapped water-soluble fluorophore, pyranin. With a pH gradient imposed upon SUV preloaded with FA, the rate of flip-flop of saturated very long chain FA (C20:0, C:22:0, and C24:0) was shown to be fast (t1/2 < 1 s); previously, we showed by stopped flow measurements that flip-flop of long chain (14-18 carbons) FA is very fast [t1/2 < 10 ms; Kamp, F., et al. (1995) Biochemistry 34, 11928-11937]. The rates of dissociation of FA from SUV were evaluated by incorporating FA into donor vesicles and measuring transfer to acceptor vesicles. The transfer was followed by changes in internal pH of either donor or acceptor vesicles with stopped flow (C14:0, C16:0, C17:0, C18:0, C18:1, and C18:2) or on-line (C20:0, C22:0, and C24:0) fluorescence. All FA showed a single-exponential transfer process that was slower than the lower limits established for the rate of flip-flop, with t1/2 of dissociation ranging from 20 ms for C14:0 to 1900 s for C24:0. The pseudo-unimolecular rate constant (koff) for dissociation of C14:0 to C26:0 showed a 10-fold decrease for each addition of two CH2 groups to the acyl chain and a delta (delta G) of -740 cal/CH2. The dissociation rate constants for oleic acid (18:1) and linoleic acid (18:2) were 5 and 10 times faster, respectively, than that of C18:0. The rates of dissociation for typical dietary FA are sufficiently rapid that complex mechanisms (e.g. protein-mediated) may not be required for their desorption from biological membranes. The very slow dissociation rates for C24:0 and C26:0 may accentuate their pathological effects in diseases in which they accumulate in tissues.