Lactose present in whey UF-permeates was hydrolysed by an immobilised enzyme reactor and the formation of monosaccharides (glucose + galactose) and oligosaccharides was monitored The enzyme used was beta-galactosidase from A. oryzae immobilised in a porous film. The reactor, run in the flow-through mode, allowed large conversions at short residence times (60% conversion in I min). The conversion to oligosaccharides as a function of the reaction time (or degree of conversion) reaches a maximum and then declined as oligosaccharides were converted back to mono- and disaccharides. The higher the initial lactose concentration the higher the conversion to oligosaccharides and these maxima appear at higher degrees of conversion. Some trials were carried out on the concentration of the oligosaccharides present in the hydrolysates by means of membrane filtration (nanofiltration).