The phase behavior of mixtures of dimyristoylphosphatidylcholine (DMPC) with N-palmitoylsphingosinephosphorylcholine (C16SHP) has been investigated in both small unilamellar and large multilamellar vesicles. The steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) has been used to detect temperature-induced structural changes in these membranes. In addition, electron microscopy has revealed vastly different fracture-face morphologies for large multilamellar vesicles "jet-frozen" from different temperatures. These data have been interpreted in terms of proposed phase diagrams for this lipid mixture. The shapes of the proposed phase diagrams have led us to conclude that phosphatidylcholine and sphingomyelin species of similar acyl chain length mix freely in both highly curved and uncurved bilayers, except at temperatures at which both lipids are in low-temperature, ordered phases. In addition, the similarity of these phase diagrams to phase diagrams for analogous mixtures of pure phosphatidylcholines suggested that sphingomyelin and phosphatidylcholine suggested that sphingomyelin and phosphatidylcholine species might substitute for each other in supporting the lamellar phase necessary for each other in supporting the lamellar phase necessary to cell membrane structure. Finally, the anisotropy of DPH fluorescence was found to be essentially invariant with sphingomyelin content at temperatures just above and below the solid--liquid phase separation in small unilamellar vesicles. This demonstrates that the sphingomyelin backbone, per se, does not order the membrane bilayer. These results are discussed in terms of the possible role of sphingomyelin in controlling acyl chain order within mammalian cell membranes.