Phospholipids are a major component of biological membranes. In excess water, phospholipids may self-assemble into fully hydrated lamellae which, upon heating, may undergo the gel to liquid-crystalline phase transition at the characteristic temperature, Tm. Our present knowledge about the Tm values for various phospholipids is far from complete, although it is necessary to know the Tm value for preparing liposomes. In this study, we have derived empirically a general expression of Tm = 154.2 + 2.0(delta C) - 142.8(delta C/CL) - 1512.5(1/CL) in which two apparent structural parameters, delta C and CL, of a phosphatidylcholine molecule and their ratio, delta C/CL, are applied to estimate the Tm value of the phosphatidylcholine bilayer in excess water. The parameter delta C is the effective chain-length difference, in C-C bond lengths, between the two acyl chains for the phosphatidylcholine molecule in the gel-state bilayer, and CL is the effective length of the longer of the two acyl chains, also in C-C bond lengths. A figure containing 163 calculated Tm values is presented, and this information will be useful as a guide for designing experiments.