The potential for osmotic distillation flux enhancement in grape juice concentration by ultrafiltration pretreatment has been investigated using ultrafiltration membranes with a range of pore diameter/MWCO values. Juice viscosities were determined over the concentration range to assist with interpretation of the flux data. Ultrafiltration using membranes with pore diameters of 0.1 mu m or less resulted in appreciable osmotic distillation flux increases over that observed for juice not subjected to ultrafiltration. These flux increases have been attributed to a reduction in the viscosity of the concentrated juice-membrane boundary layer as the result of protein removal. HPLC measurements showed that the normal concentration of fermentable sugars in standard 68 degrees Brix concentrate can be achieved at a lower Brix value with UF permeate, thereby providing a possible means of reducing the handling of highly viscous streams. Ultrafiltration also resulted in an increase in juice surface tension with a consequent reduction in the tendency for membrane wet-out to occur. (C) 2000 Elsevier Science B.V. All rights reserved.