A mathematical model, describing the extraction of pectin with a first order chemical reaction in the solid phase was proposed. A numerical solution was obtained, which is compared with the experimental data for the increase of the pectin concentration in the liquid phase. A good fit with experimental data was demonstrated. As a result the values of the rate coefficient of the acidic hydrolysis (9.8 x 10(-4) and 9.7 x 10(-4) s(-1)), the coefficient of effective diffusion in the particles (6.57 x 10(-11) and 7.9 x 10(-11) m(2)/s) and the external mass transfer coefficient (6.62 x 10(-5) m/s for particles with mean radius of 0.25 x 10(-3) m and 2.56 x 10(-5) m/s for particle with mean radius of 0.8125 x 10(-3) m) were estimated.