Monte Carlo (MC) simulations have been carried out for supercritical mixtures of Lennard-Jones (LJ) fluids to investigate the unique behavior of excess thermodynamic functions in supercritical region obtained experimentally. It is reported that constant pressure excess molar enthalpies H-p(E) and excess molar volumes V,E take large absolute values and show complex concentration dependencies in such regions. In this paper, H-p(E) and V-p(E) have been calculated for a total of 12 kinds of mixtures, each having different combinations of energy and size parameters and the combining rule for unlike interactions. It is found that such a unique thermodynamic behavior of real systems in their supercritical region can be reproduced, at least qualitatively, by simple model systems. It is also found that the unique thermodynamic behavior is closely related to the equation-of-state of pure components in their supercritical region. In order to estimate the contribution to the constant pressure excess thermodynamic functions from the equation-of-states of pure components, we have developed a data conversion method by which we can calculate a new function, excess internal energy at constant volume from H-p(E), V-p(E) and equation-of-states of purl components. The converted excess internal energies at constant volume for the present model systems become quite simple; that is, their magnitudes are reduced to be small and the concentration dependencies become symmetrical. Further analysis indicates that the large differences of molar volume and internal energy between pure components and large V-p(E) are responsible for the unique behavior of excess functions at constant pressure. The data conversion method is also applied to the experimental data for the CO2+C2H6 system in the supercritical region. The obtained excess internal energies at constant volume also show similar simple behavior as those for the model systems. Furthermore, it is found that the complex dependence of experimental H-p(E) on pressure is also closely related to equation-of-states of pure components. (C) 1997 American Institute of Physics.