This thesis describes the ionic strength and ion binding effects on the oxidation reduction properties of cytochrome c and its lysine modified derivatives. Cytochrome c has been modified in two different ways: a complete modification of all lysine residues and specific modification of one lysine residue. Some properties of the modified derivatives are described. Ion binding properties of cytochrome c and its lysine modified derivatives were studied by measuring the apparent equilibrium constant of the reaction between the; ferri- form of the protein and potassium ferrocyanide. It was found that unmodified cytochrome c binds one cation (K+, Na+) per molecule, and binding is much stronger to the reduced form of the protein. Binding of cations is not changed upon modification of the lysine residues. For binding of the chloride, there are two binding sites on the cytochrome c molecule, and the binding is much stronger to the oxidized form of the protein. It was shown that upon the modification of the lysine residues in either way the binding of chloride was considerably changed. It was concluded that one of these two binding sites for chloride on cytochrome c involves lysine residue, probably the residue number 13. Thesis Master of Science (MSc)