Sphingolipids are key lipid regulators of cell viability: ceramide is one of the key molecules in inducing programmed cell death (apoptosis), whereas other sphingolipids, such as ceramide 1-phosphate, are mitogenic. The thermotropic and structural behavior of binary systems of N-hexadecanoyl-D-erythro-ceramide (C(16)-ceramide) or N-hexadecanoyl-D-erythro-ceramide-1-phosphate (C(16)-ceramide-1-phosphate; C(16)-C1P) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with DSC and deuterium nuclear magnetic resonance ((2)H-NMR). Partial-phase diagrams (up to a mole fraction of sphingolipids X = 0.40) for both mixtures were constructed based on DSC and (2)H-NMR observations. For C(16)-ceramide-containing bilayers DSC heating scans showed already at X(cer) = 0.025 a complex structure of the main-phase transition peak suggestive of lateral-phase separation. The transition width increased significantly upon increasing X(cer), and the upper-phase boundary temperature of the mixture shifted to approximately 65 degrees C at X(cer) = 0.40. The temperature range over which (2)H-NMR spectra of C(16)-ceramide/DPPC-d(62) mixtures displayed coexistence of gel and liquid crystalline domains increased from approximately 10 degrees for X(cer) = 0.1 to approximately 21 degrees for X(cer) = 0.4. For C16-C1P/DPPC mixtures, DSC and (2)H-NMR observations indicated that two-phase coexistence was limited to significantly narrower temperature ranges for corresponding C1P concentrations. To complement these findings, C(16)-ceramide/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and C16-C1P/POPC mixtures were also studied by (2)H-NMR and fluorescence techniques. These observations indicate that DPPC and POPC bilayers are significantly less perturbed by C(16)-C1P than by C(16)-ceramide and that C(16)-C1P is miscible within DPPC bilayers at least up to X(C1P) = 0.30.