Movement of free fatty acids (ffa) between small unilamellar vesicles (SUV) was studied by measuring the transfer of fluorescent n-(9-anthroyloxy)-labeled analogues (AOffa) between donor and acceptor vesicles. Donors were composed of egg phosphatidylcholine (PC) loaded with 1-2 mol % AOffa, and acceptors were egg PC containing 10-12 mol % N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE). The fluorescence of AO added directly to acceptor SUV was greater than 98% quenched by energy transfer to NBD. Thus, AOffa movement from donor to acceptor was monitored by the time-dependent decrease in AO fluorescence. The transfer of the short-chain AOffa, although too fast to be resolved by the methods used here, is consistent with studies that find transfer rates on the order of milliseconds and kinetics which are first order. In contrast, transfer rates for the long-chain AOffa are more than 2 orders of magnitude slower, and the kinetics of the transfer process are best described by the sum of two exponentials plus a constant. The ffa ionization state was also found to be an important determinant of transfer rate. The charged species transferred an average of 10-fold faster than the protonated ffa. The ffa pKa in the membrane is 9, as calculated from the pH dependence of transfer. Similar to results found for other lipids, long-chain AOffa are transferred via water rather than a collision-mediated process. The aqueous phase route of AOffa intermembrane transfer is indicated by the lack of effect on transfer of large alterations in the product of donor and acceptor phospholipid concentrations. Moreover, the transfer rate is decreased as [NaCl] is increased from 0.1 to 4 M. This effect of ionic strength is probably due not only to a decrease in the aqueous phase partition of the AOffa but also to an alteration in bilayer structure, as measured by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. The observed kinetics are consistent with a model in which the transfer involves two steps: transbilayer movement between the inner and outer bilayer leaflets, followed by transfer from the outer leaflet to the aqueous phase (off rate). Within the framework of this model, the observed slow rate is primarily determined by the rate of transbilayer movement, and the observed fast rate is approximately equal to the off rate. The off rate is about 10-fold faster than the rate of transbilayer movement.