Diacyl phosphatidylcholines with n-C13, -C15, -C17 and -C19 saturated acyl chains have been synthesized and their phase transitions in the presence of excess water monitored by differential thermal analysis. The C15-, C17- and C19-diacyl species show gel to liquid-crystalline transitions and pretransitions like those of the even-chain phosphatidylcholines. A plot of the main phase transition temperature, Tc, vs. acyl chain length is a smooth curve on which the Tc values of both odd- and even-chain species fall, while a similar plot for the pretransition temperature, Tp, shows significant alternation of Tp between odd- and even-chain species. Consideration of these results in terms of the physical basis of the odd-even alternation of phase transition temperatures in homologous series of paraffinic compounds suggests that the acyl chains of disaturated phosphatidylcholines are tilted with respect to the bilayer normal below Tp but become perpendicular to the bilayer surface above the pretransition temperature.