Bilayer phase transitions of dioctadecyldimethylammonium bromide (2C(18)Br) and chloride (2C(18)Cl) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. The 2C(18)Br bilayer membrane showed different kinds of transitions depending on preparation methods of samples under atmospheric pressure. Under certain conditions, the 2C(18)Br bilayer underwent three kinds of transitions, the metastable transition from the metastable lamellar crystal (L-c(2)) phase to the metastable lamellar gel (L-beta) phase at 35.4 degrees C, the metastable main transition from the metastable L-beta phase to the metastable liquid crystalline (L-alpha) phase at 44.5 degrees C, and the stable transition from the stable lamellar crystal (L-c(1)) phase to the stable L-alpha phase at 52.8 degrees C. On the contrary, the 2C(18)Cl bilayer underwent two kinds of transitions, the stable transition from the stable L-beta phase to the stable L-alpha phase at 19.7 degrees C and the stable main transition from the stable L-beta phase to the stable L-alpha phase at 39.9 degrees C. The temperatures of the phase transitions of the 2C(18)Br and 2C(18)Cl bilayers were almost linearly elevated by applying pressure. It was found from the temperature (T)-pressure (p) phase diagram of the 2C(18)Br bilayer that the T-p curves for the main transition and the L-c(1)/L-alpha transition intersect at ca. 130 MPa because of the larger slope of the former transition curve. On the other hand, the T-p phase diagram of the 2C(18)Cl bilayer took a simple shape. The thermodynamic properties for the main transition of the 2C(18)Br and 2C(18)Cl bilayers were comparable to each other, whereas those for the L-c(1)/L alpha transition of the 2C(18)Br bilayer showed considerably high values, signifying that the L-c(1) phase of the 2C(18)Br bilayer is extremely stable. These differences observed in both bilayers are attributable to the difference in interaction between a surfactant and its counterion.