This paper is a contribution to mathematically describing sugar syrup purification, i.e. ultrafiltration of non-sucrose compounds, based on the experimental data, acquired in a series of batch experiments on flat polyethersulfone cross-flow membranes. Experiments were performed to final concentration factors not less than 1.5 and not greater than 2.3, with flow rates 0.2 and 0.4 m(3) h(-1), at the temperatures 30 and 60 degreesC and for the transmembrane pressures I and 4 bar. The suggested mathematical model enables prediction of separation time if the permeate flux models as well as the initial and final concentrations of undesired non-sucrose compounds are known. In this paper, the flux models are proposed as functions of concentration factor (CF), flow rate (Q), temperature (t) and transmembrane pressure (Deltap) as independent variables. So, the following one-, two- and three-variable functions are suggested: J(-1)(CF), J(-1)(CF, Q), J(-1) (CF, t), J(-1) (CF, Deltap), J(-1) (CF, Q, t) and J(-1) (CF, Q, Deltap). By the regression analysis method, parameters in the flux models are determined and they are used for the calculation of process duration. It is concluded that the predicted times underestimate the real times by 10-40% in particular cases. One can assume that the incomplete rejection of nonsucrose compounds by polyethersulfone membranes as well as the loss of sugar due to accumulation at the membrane is responsible for process prolongation. (C) 2003 Elsevier Ltd. All rights reserved.