Monolayers of the naturally occurring cationic lipid sphingosine and its mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were studied using a Langmuir balance. More specifically, we measured the force-area (pi-A) isotherms and determined the monolayer dipole potential T as a function of the mole fraction of sphingosine (X-Sph) with and without a charge-saturating concentration of DNA in the subphase. Both sphingosine and POPC exhibited smooth compression isotherms, indicating their monolayers to be in the liquid expanded state. Even low contents (X-Sph = 0.05) of sphingosine in a POPC monolayer condensed the film dramatically, by 20% at 20 mN/m. This effect is suggested to reflect a reorientation of the P--N+ dipole of the POPC headgroup, (Saily, V. M. J.; Ryhanen, S. J.; Holopainen, J. M.; Borocci, S.; Mancini, G.; Kinnunen, P. K. J. Biophys. J. 2001,81,2135), in keeping with a simultaneous and pronounced increase in T. Mixed monolayers of sphingosine and POPC exhibited three critical mole fractions X-Sph of sphingosine, viz., 0.25,0.6, and 0.83, at which the area/molecule reached a local minimum, followed by a pronounced expansion of the film. This suggests energetically favorable ordering, which allows the positively charged sphingosines to maximize their distance, so as to minimize the Coulombic repulsion. The presence of DNA affected the mixed POPC/sphingosine monolayers differently depending on the constituent lipid stoichiometry, yet the same discontinuities were evident as in the presence of DNA.