The effect of cationic surfactants on the bilayer phase transition of phospholipid vesicles was studied. Two types of acidic phospholipids, phosphatidic acid and phosphatidylglycerol, with different acyl chain length were examined. Cationic surfactants were aikyltrimethylammonium bromides with different aikyl chain length. The cationic surfactants with short chain length induced a monotonous decrease in the transition temperature, T, with increasing concentration, while those with long chain length exhibited a biphasic effect on T, i.e., increase in T at low concentrations and decrease in T at high concentrations. The surfactant chain length at which the biphasic effect on T appeared depended on the lipid acyl chain length; the longer lipid acyl chain needed the longer surfactant chain to bring about the biphasic effect. This behavior of the bilayer phase transition was common to acidic phospholipids regardless of the type of the lipid head groups. The complicated response of T to additives was explained by introducing interactions between the lipid and the surfactant in the membrane phase. According to this model, the behavior of T with respect to the additive concentration is determined by the combination of differences in the lipid-additive interaction energy and the free energy of additives between solid-gel and liquid-crystalline membranes.