The effects of flow direction of solvent (CO2) on the fluid-to-particle mass transfer were studied for packed beds of sintered porous pellets of two sizes (diameters of 1 and 2 cm) at conditions supercritical with respect to carbon dioxide using toluene and 1,2-dichlorobenzene as the impregnating solutes. Dynamic extraction experiments were performed in the laminar flow regime (Re = 8-90) where both free and forced convection modes of mass transfer were found to be significant. Measured mass transfer coefficients showed an almost linear dependence on the Reynolds number (Re-approximate to 0.9), Downflow of fluid had a strong effect on accelerating extraction rates, in particular at lower Reynolds numbers and for conditions near the critical point of CO2, where natural convection is dominant. Experimental mass-transfer coefficients were well correlated using a single general equation that accounts for both modes of mass transfer (free and forced convection) as well as for upflow or downflow operation (i.e,, with gravity opposing or assisting forced convection).