We observed that the mutual diffusion coefficients of benzene in supercritical carbon dioxide exhibited a maximum at pressures above the critical pressure of the mixture, and a strong concentration dependence near the critical point. These surprising experimental results can be explained by replacing the concentration gradient of diffusion with a chemical potential gradient as suggested fi om irreversible thermodynamics. Then, it can be understood that diffusion coefficients have a maximum value at the critical point of a mixture. The Peng-Robinson equation of state was used to illustrate these relationships.