In this paper, a statistical thermodynamic model is presented to describe mathematically the retention behaviour of ion exchange chromatography. Emphasis is placed on the relation between the retention volume and the ion-potential of the solute ion and the following formula is obtained V-R = V-0 (1 + 1-alpha/alpha K) InK = c - (a zeta(A) + b zeta(A)(2))/RT -Z(A)/Z(B) InCB where alpha is the porosity of the column; V-R and V-0 are the retention volume and the dead volume respectively; zeta(A) is the ion-potential of the solute ion A, z(A) and z(B) are the charges of the ions A and B; a and b are constants.