The binding of cytochrome c to phospholipid vesicle is studied by using centrifugation method at various pH and ionic strength. The binding curve of cytochrome c to liposome is hyperbolic, leading to the suggestion that the vesicle has identical and independent binding sites of limited numbers and is bound with the same microscopic dissociation constant. The binding is significant at low ionic strength, but it``s pH dependence is complicated. At high ionic strength, the binding is not observed above pH 4.0. Blow this pH, the extent of binding is increased with decreasing pH. The binding is reversible at pH9.0, but at pH3.0, part of cytochrome c remained bound to the vesicles upon dilution. It appears that the cytochrome c is bound to the vesicles through electrostatic interaction at high pH between positively charged protein and negatively charged polar head group of liposome while at low pH, the hydrophobic interaction may be involved.