The bilayer phase transitions of dialkyldimethylammonium bromides (2C(n)Br; n = 12, 14, 16) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. Under atmospheric pressure, the 2C(12)Br bilayer membrane underwent the stable transition from the lamellar crystal (L(c)) phase to the liquid crystalline (L(alpha)) phase. The 2C(14)Br bilayer underwent the main transition from the metastable lamellar gel (L(beta)) phase to the metastable L(alpha) phase in addition to the stable L(c)/L(alpha) transition. For the 2C(16)Br bilayer, moreover, three kinds of phase transitions were observed: the metastable main transition, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable L(alpha) phase, and the stable lamellar crystal (L(c(1)))/L(alpha) transition. The temperatures of all the phase transitions elevated almost linearly with increasing pressure. The temperature (T)-pressure (p) phase diagrams of the 2C(12)Br and 2C(14)Br bilayers were simple, but that of the 2C(16)Br bilayer was complex; that is, the T-p curves for the metastable main transition and the L(c(2))/L(alpha) transition intersect at ca. 25 MPa, which means the inversion of the relative phase stability between the metastable phases of L(beta) and L(c(2)) above and below the pressure. Moreover, the T-p curve of the L(c(2))/L(alpha) transition was separated into two curves under high pressure, and as a result, the pressure-induced L(c(2P)) phase appeared in between. Thermodynamic quantities for phase transitions of the 2C(n)Br bilayers increased with an increase in alkyl-chain length. The chain-length dependence of the phase-transition temperature for all kinds of transitions observed suggests that the stable L(c(1))/L(alpha) transition incorporates the metastable L(c(2))/L(alpha) transition in the bilayers of 2C(n)Br with shorter alkyl chains, and the main-transition of the 2C(12)Br bilayer would occur at a temperature below 0 degrees C.