Experimental data and mathematical models are presented for extraction from plants in a continuous countercurrent screw extractor operating with solvent recycling. The working process of the device was analyzed for two kinetically different solid-liquid systems: Geranium macrorhizum L. -water and Nicotiana tabacum L. -water. A dimensionless convection-diffusion model, adapted for the relevant flow configuration, was solved numerically under dynamic conditions. From independent experiments in a periodically stirred vessel and in a continuous screw extractor, the model parameters (effective diffusivity, mass-transfer coefficient, and axial dispersion) were obtained by comparing the model solutions to the experimental data. It was found that, for systems containing dilute solutions at high solvent velocity with an internal-diffusion-controlled process (Bi much greater than 40), a simplified perfect-mixing approximation successfully fits the experimental data for the larger particle sizes studied.