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Ceramide-1-phosphate (Cer-1-P), one of the simplest of all sphingophospholipids, occurs in minor amounts in biological membranes. Yet recent evidence suggests important roles of this lipid as a novel second messenger with crucial tasks in cell survival and inflammatory responses. We present a detailed description of the physical chemistry of this hitherto little explored membrane lipid. At full hydration Cer-1-P forms a highly organized subgel (crystalline) bilayer phase (L_c) at low temperature, which transforms into a regular gel phase (L_β) at ∼45°C, with the gel to fluid phase transition (L_β–L_α) occurring at ∼65°C. When incorporated at 5mol % in a phosphatidylcholine bilayer, the pK_a2 of Cer-1-P, 7.39±0.03, lies within the physiological pH range. Inclusion of phosphatidylethanolamine in the phosphatidylcholine bilayer, at equimolar ratio, dramatically reduces the pK_a2 to 6.64±0.03. We explain these results in …