The concentration dependence of diffusion coefficients for naphthalene in supercritical carbon dioxide at 308.2 K was measured by a pseudo steady-state solid dissolution method. The experimental diffusion coefficients were compared with the calculated results by the Darken equation including a thermodynamic factor and tracer diffusion coefficients of supercritical carbon dioxide and naphthalene. The thermodynamic factor in the Darken equation was determined by using several cubic equations of state. The calculated results by the Darken equation represent the concentration dependence of the experimental diffusion coefficients.