The kinetics and structural changes in the solid were studied for solid-liquid extraction from plants. Two extraction systems were investigated: tobacco leaves (Nicotiana tabacum L.)-water, and oak bark (Quercus frainetto Ten)-water. The mathematical model of the process, accounting for variable effective diffusivity and porosity, was stated and solved numerically. The solution was checked with experimental kinetics and porosity data to determine the effective diffusion coefficient. A very good correspondence was found between this value and the results obtained with the regular regime and the standard function methods.