The influences of structural alterations of sphingomyelin (SM) on its interactions with cholesterol (chol) and on ordered phase formation were examined by density measurements and surface pressure vs. molecular area isotherm measurements. In addition, we quantitatively characterized the ordered phase formed in each SM and chol binary mixture on the basis of the molecular compressional modulus of SM (Cmol-1). Density measurements demonstrated that the ordered phase formation in threo- SM (tSM)/chol and dihydrosphingomyelin (DHSM)/chol binary bilayers shows similar chol concentration-dependency to that of natural erythro-SM (eSM)/chol bilayers; the ordered phase formation was completed in the presence of 25 mol% chol. In contrast, SM bearing a triple bond in the place of a double bond (tripleSM) required a greater concentration of chol to completely transform the bilayer into the ordered phase (at 40mol% chol). Surface pressure vs. molecular area isotherms showed that the DHSM molecule (Cmol-1=290mN/m) is more rigid than eSM (Cmol-1=240mN/m) above 30 mol% chol (in the ordered phase), although these values are similar (140- 150mN/m) in the absence of chol (liquid condensed phase). Most likely, the DHSM/chol mixture forms a more ordered membrane than the eSM/chol mixture does. Moreover, in the absence of chol, the rigidity of the tripleSM molecule (Cmol-1=250mN/m) is significantly higher as compared with that of the eSM molecule (Cmol-1=150mN/m), which is probably due to the presence of a triple bond.