To clarify the influence of cis and trans double bonds on conformational disordering of unsaturated hydrocarbon-chain of lipids in bilayer formed in the binary system with water, calorimetric study was conducted for systems of unsaturated monoacylglycerols; monoolein (MO), monovaccenin (MV), and monoelaidin (ME). Heat capacities of the binary systems were measured by adiabatic calorimetry. The observed entropies of transition (Delta(trs)S) from lamellar phase (L(alpha)) to fluid isotropic (FI) phase are very small and depend on lipids: Delta(trs)S of MO/water, MV/water, and ME/water were ca. 0.8 J K(-1) (mol of lipid) (-1) ca. 0.9 J K(-1) (mol of lipid) (-1) and ca. 0.4 J K(-1) (mol of lipid) respectively. These show that the conformational disordering of the hydrocarbon chain over gauche and trans conformations is suppresed in L(alpha) phase. Through the comparison of Delta(trs)S among the binary systems, the suppresed conformational disordering of hydrocarbon chain in the bilayer is discussed.