Physical properties of binary mixtures of dipalmitoylphosphatidylcholine and yeast phosphatidylinositol were studied by ESR analysis using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and lipid spin probes, freeze-fracture electronmicroscopy and particle microelectrophoresis, and they were compared with those of phosphatidylcholine/bovine brain phosphatidylserine mixtures. The phase diagram of the binary mixtures of dipalmitoylphosphatidylcholine and phosphatidylinositol was obtained from the thermal features of TEMPO spectral parameter in the lipid mixtures. The phase diagram provided evidence that these two phospholipids in various combinations were miscible in the crystalline state. The addition of 10 mM Ca2+ slightly shifted the phase diagram upward. TEMPO titration of the binary mixture of dipalmitoylphosphatidylcholine and bovine brain phosphatidylserine revealed that 10 mM Ca2+ caused the complete phase separation of this lipid mixture. Studies of phase separations using phosphatidylcholine spin probe manifested that 10 mM Ca2+ induced almost complete phase separation in egg yolk phosphatidylcholine/bovine brain phosphatidylserine mixtures but only slight phase separation in egg yolk phosphatidylcholine/yeast phosphatidylinositol mixtures. However, some phase changes around the fluidus and the solidus curves were visualized by the freeze-fracture electronmicroscopy. The molecular motion of lipid spin probe was decreased by the addition of Ca2+ in the liposomes containing phosphatidylinositol. The temperature dependence of electrophoretic mobility was also examined in the absence and presence of 1 mM Ca2+. Liposomes of dipalmitoylphosphatidylcholine-phosphatidylinositol (90 : 10, mol/mol) exhibited a clear transition in the thermal features of electrophoretic mobilities. Raising the phosphatidylinositol content up to 25 mol% rendered the transition broad and unclear. The addition of 1 mM Ca2+ decreased the electrophoretic mobility but did not change its general profile of the thermal dependence. These results suggest that the addition of calcium ions induced a small phase change in the binary mixture of phosphatidylcholine and phosphatidylinositol while Ca2+ causes a remarkable phase separation in phosphatidylcholine/phosphatidylserine mixture. The physical role of phosphatidylinositol is discussed related to the formation of diacylglycerol.