The gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was measured in the presence of sodium octanoate (SO) (pH 3 and 10) and sodium perfluorooctanoate (SPFO) (pH uncontrolled) by monitoring the scattered light intensity of the vesicle suspension. The phase transition temperature, Tin, decreased linearly with the concentration of added SO within the measured concentration range; the uncharged form of SO (pH 3) was much more effective for the depression of T m than the charged form (pH 10). On the other hand, with increasing SPFO concentration, levelling off of T was observed after depression at an initial stage. From the depression of T, the partition coefficients, K, of these surfactants between bulk solution and DPPC vesicle membrane were estimated and compared with those obtained previously for other surfactant systems. The value of K for charged SO fell on thg straight line of log K vs. N plot for anionic surfactants, where N is the carbon number of the hydrocarbon chain of surfactants, whereas K for uncharged SO showed a large positive deviation from the straight line of the plot for non-ionic surfactants. The latter suggested that some specific interaction, presumably hydrogen bond formation, may act between the protonated carboxyl group of SO and the lipid head group. The K value estimated for SPFO was much larger than that for charged SO. This difference in the affinity for the lipid bilayer between fluorocarbon surfactant and hydrocarbon surfactant may be attributed to the difference in their hydrophobicity.