The current work is concerned with obtaining a new empirical model for the time-variable effective diffusion coefficient (D-eff) by extraction from plant materials. It has been applied for describing a solid liquid extraction from Cotinus coggygria and compared with experimentally obtained extraction kinetics. The experiments have been performed by changing the liquid-solid ratio (xi = 0.01, 0.02, and 0.03 m(3) kg(-1)), temperature (20, 40, and 50 degrees C), and extraction solvents (water, 50% ethanol in water, and 70% ethanol in water). A nonlinear seven-parametric model for D-eff is obtained in order to represent the influence of those parameters on the extraction process and D-eff, respectively. The parameters of the model are obtained by a nonlinear regression of D-eff values calculated using the method of regular regime. All experimental conditions have been taken into consideration. A very good coincidence between experimental and empirically obtained data has been found. This is a proof for the ability of the function to describe an extraction process. It can be used to estimate and control the process.