Mutual diffusion coefficients of benzene in supercritical carbon dioxide are determined at 313.15 K from 8 to 20 MPa using the Taylor dispersion method, A 10 m diffusion tube was oriented in a horizontal position. A UV detector was used to determine solute concentration profiles. We find that the diffusion coefficients have maxima depending on injection volumes and appear to approach zero for 5.7 and 13.1 mu l injection volumes near the critical pressure of carbon dioxide, although their values decrease monotonically with pressure for 0.7 and 1.0 mu l injection volumes. Based on irreversible thermodynamics, calculation from an equation of state reveals qualitatively anomalous behavior, Below 8 MPa, the decay in concentration distribution due to fluctuation in the critical region is immeasurable with UV due to intense light scattering but can be measured with a flame ionization detector.