The rates of hydration of naturally occurring fatty acids bound to unilamellar vesicles of dimyristoylphosphatidylcholine were measured by following the rate of quenching of the inherent fluorescence of albumin. Rates of hydration of fatty acids bound to albumin could be estimated from the same data. The data show that these rates depend on the chain length and unsaturation of the fatty acid. Increasing chain length diminishes the rate of hydration whereas increasing unsaturation increases this rate. Rates of hydration of fatty acids bound to lipid vesicles appear to be rapid enough to account for intracellular movement between compartments in the absence of carrier proteins. It is uncertain whether this is true for hydration of fatty acids bound to albumin. Rates for this process are about 100-300 times slower vs. rates of hydration of fatty acids bound to lipid vesicles.