In the present work the ultrafiltration (UF) of orange press liquor was analyzed by means of the response surface methodology (RSM). The liquor was clarified by using polysulphone hollow fiber membranes with a molecular weight cut-off (MWCO) of 100 kDa. The influence of different operating conditions, such as transmembrane pressure (TMP), axial feed flow rate (Q(f)) and temperature (T) on the membrane rejection towards polyphenols and the recovery of antioxidant compounds in the permeate stream was investigated. The experimental operating conditions were selected within the following ranges: TMP 0.2-1.4 bar, temperature 15-35 degrees C, and feed flow-rate 85-245 L/h. A total of 30 ultrafiltration experiments were performed. Judged by the lack-of-fit criterion, the analyses of variance (ANOVA) showed the regression model to be adequate. From the regression analyses, the membrane rejection towards polyphenols and total antioxidant activity (TAA) were expressed with quadratic equations of TMP, Q(f) and T. The predicted rejection towards polyphenols and TAA in the clarified liquor from the regression model were presented in 3D surface plots. Quadratic terms of TMP, T and Q(f) showed significant (p > 0.05) influence on the polyphenols rejection. Results indicated a strong interaction effect of TMP with T and Q(f). For the TAA the quadratic effect of T was the most significant. In this case a strong interaction effect of TMP with Q(f) was detected. The desirability function approach was applied to analyze the regression model equations in order to optimize the recovery of antioxidant compounds in the permeate stream. The optimized operating conditions were 0.2 bar, 19.85 degrees C and 244.64 L/h (maximum TAA, minimum polyphenols rejection). (C) 2012 Elsevier B.V. All rights reserved.