In this work, a study about the conditions that maximize the phenol yields and the antioxidant capacity of extracts from grape byproducts obtained by continuous extraction is undertaken. Optimization was carried out by response surface methodology, using ethanol as a solvent and 50 degrees C as extraction temperature. For the particular experimental setup used, the optimal values of three critical extraction variables were found to be: 2 mL/mm of flow rate (values varied between 2 and 3 mL/mm), 2.5 g of sample quantity (between 2.5 and 7.5 g) and 0.5 mm of particle size (between 0.5 and 5.5 nun). Since diffusion of the dissolved solute within the solid into the solvent was found to be the rate limiting step, the process can be described by Fick's second law. Higher values of phenol yields and antioxidant capacity corresponded to extraction conditions in which effective diffusivity/particle size ratio was higher. In this case, an effective diffusivity value of 105.49 x 10(-14), m(2) s(-1) was attained. (c) 2005 Elsevier Ltd. All rights reserved.