In this paper, the supercritical-fluid extraction (SCFE) of a packed bed of β-naphthol-impregnated porous pellets was studied. An increasing number of industrial SCFE processes involve the extraction of a solute retained within a porous matrix, usually in the form of seeds or irregular grains. The interest in high-pressure extraction is due to certain advantages of dense gases and near-critical solvents over conventional liquid solvents. In this study, modified carbon dioxide was the fluid studied. The effects of temperature, pressure, fluid velocity, particle size, and gravity were experimentally studied using carbon dioxide, pure or mixed with varying amounts of toluene (6%, and 10%). For the solute, β-naphthol, the solubilities in SC carbon dioxide mixtures (from 0 to 10% toluene) were available from separate experiments. The dispersed plug-flow model was used to describe the nonideal flow. Fitting the experimental data with the model solution allowed the measurements of the fluid-to-particle mass transfer coefficient, the intraparticle diffusivity, and the axial dispersion coefficient (the latter in terms of the axial Peclet number). The influence of cosolvent concentration on the three transport parameters, which were not available so far, is presented.