Positive excess molar Gibbs energies were detected in liquid mixtures of (Ar-36+Ar-40) using a high-accuracy double differential manometric technique. The vapor pressure differences between a mixture of (Ar-36+Ar-40) and Ar-40 and between Ar-36 and Ar-40 were measured simultaneously with the vapor pressure of Ar-40. Four different mixtures were selected and 80 temperatures were chosen in the 85-96 K temperature range. The results show that the vapor pressure of an equimolar liquid mixture is greater than what would be expected if the solution were strictly ideal, by approximately 2.5 Pa in the 85-96 K temperature range. The calculated excess Gibbs energy for the equimolar mixture at 85 K is G(1/2)(E)=(0.021 +/-0.004) J mol(-1). For the excess molar enthalpy a value of H-1/2(E)=(0.12 +/-0.04) J mol(-1) was found. The corresponding deviations from Raoult's law are therefore extremely small (3 to 4 orders of magnitude smaller than those typically found in binary mixtures of nonisotopic constituents). The evidence of nonideal behavior in such a fundamental and simple system is relevant for the interpretation of the properties of liquid mixtures of more general importance. The pitfalls of the determination of activity coefficients through the comparison between vapor pressure isotope effect data and liquid-vapor isotope fractionation factors are discussed in some detail. The results were used to test three theoretical models, namely, the vdW-1 fluid theory, the 1cLJ perturbation theory, and the statistical theory of isotope effects in mixtures. While the first two proved to be inadequate, the isotope effect theory agrees reasonably well with experiment. The excess Gibbs energy of monatomic isotopic mixtures are related with the molar volume isotope effect and with the mean Laplacian of the potential energy in the liquid phase.