Synchrotron X-ray studies and surface pressure versus molecular area (pi-A) isotherms of C18- and C20-sphingosines spread at air/water interfaces reveal unique interfacial properties with considerable differences between the two single hydrocarbon chain amino-alcohols. C20-sphingosine forms a crystalline monolayer with structural characteristics that are dominated by hydrogen bonding in the headgroup (common to its sphingolipid derivatives), whereas its natural counterpart C18-sphingosine forms a disordered liquid-like metastable monolayer and has to be spread in excess with a floating reservoir on the water surface to compensate for the high dissolution rate of molecules into the water subphase. The marginal affinity of C18-sphingosine to reside at the interface, the microcrystallization at very low densities, the corrugated monolayers it forms, and the strong interaction with the water surface are consistent with the roles that sphingolipids play in the life cycle of eukaryotic cells and as the building blocks of specialized membranes.