Liquid mixture (36Ar+40Ar) was analyzed for its nonideal behavior. The double differential manometric technique was used for the detection of excess Gibbs energies in the liquid mixture. The values of vapor pressure for the equimolar mixtures was found to be more than that of the ideal solution. The deviation of the system from the Raoult's law was very small. Activity coefficients for the vapor pressure isotope and liquid-vapor isotope were also discussed. The resultant theories were applied to three theoretical models.