An expression for the excess entropy for mixing isothermally two liquids was obtained from the entropy of mixing two fluids of rigid spheres, each at the volume of the corresponding liquid components, to form a solution of rigid spheres the volume of which was that of the liquid solution. The theoretical entropy of mixing of liquids was found to be ideal only if the diameters of the molecules are the same, the molar volumes of the components are equal, and if the volume of mixing is zero. For small deviations from the above conditions, the excess entropy of mixing was proportional to the product of the mole fractions of the components. The equation was applied to nonpolar systems; the agreement between the calculated and observed values was satisfactory.