The interactions of sphingosine (SPH) with dipalmitoylphosphatidylcholine (DPPC) and dielaidoylphosphatidylethanolamine (DEPE) have been studied by means of differential scanning calorimetry (DSC) and 31P-nuclear magnetic resonance (31P-NMR). Experiments were carried out with the fully protonated form of SPH, at pH 6.0. DSC studies showed that the main Tc transition temperature of DPPC was perturbed by the presence of SPH so that Tc of the mixture was higher than those of pure components at concentrations of SPH up to 50 mol%, with an azeotropic point at 30 mol% of SPH. At higher concentrations solid phase separations were observed from 70 to 95 mol% of SPH with an eutectic point at 90 mol% of SPH. 31P-NMR showed lamellar phases in DPPC/SPH mixtures, at all the range of concentrations. The behavior of DEPE/SPH mixtures was somewhat different since no azeotropic point was detected, the gel to liquid-crystalline transition being depressed by the presence of SPH, and an eutectic point was detected at 60 mol%. Solid phase immiscibilities were present between 50 mol% and 85 mol% of SPH. It is also remarkable that the liquid-crystalline to hexagonal HII phase transition of DEPE was only slightly shifted to lower temperatures at concentrations of SPH lower than 33 mol% of SPH but, this transition disappeared at concentrations of SPH higher than 33 mol% of SPH, so that isotropic phases were formed instead, as seen through 31P-NMR. The present results show the importance of taking into account the effects appearing in mixtures of SPH with zwitterionic phospholipids when considering their influence on the organization of biomembranes.