A new experimental apparatus based on a pseudo-steady state solid dissolution method has been designed to measure the diffusion coefficients of supercritical carbon dioxide(1)-naphthalene(2) and supercritical carbon dioxide(1)-dimethylnaphthalene isomers(2) systems. The diffusion coefficients were measured at 308.2 K. In order to establish the validity of this method and the performance of the apparatus, the experimental results of the diffusion coefficients of naphthalene in supercritical carbon dioxide were compared with the literature data measured by other methods. The decrease of diffusion coefficient of naphthalene was observed near the critical point of carbon dioxide, The diffusion coefficients of 2,6-dimethylnaphthalene (2,6-DMN) and 2,7-dimethylnaphthalene (2,7-DMN) show almost the same values. The diffusion coefficients of these isomers are about 10% lower than those of naphthalene.