Whey contains a lot of lactose, which can be easily hydrolysed by commercial enzymes. The aim of the present study was to identify the optimal parameters for the enzymatic hydrolysis of acid whey permeate and glucose-galactose syrup production. Acid whey permeate was hydrolysed using β-galactosidase preparate (NOLA TM Fit 5500, Chr. Hansen, Denmark) with activity 7200 BLU L -1 . As the enzyme is strongly inhibited at pH below 4.5, sodium bicarbonate was added to neutralize substrate pH till 6.0-6.3. The hydrolysis was carried out at 40 °C 6 hours. pH and monosaccharides concentration were monitored during the process of hydrolysis. The fermented substrate was concentrated in a vacuum evaporator at 40-60 °C, 4-8 kPa. Glucose-galactose syrup was obtained with 65 and 70% of total solids. Lactose and monosaccharides were determined by HPLC. Fermentation time influenced monosaccharides composition and concentration. After 2 hours of fermentation lactose was completely hydrolysed. Continuing fermentation, the amount of glucose was decreased due to formation of novel oligosaccharides. The study results revealed that the optimal time for acid whey permeate hydrolysis was 2 hours. It should be noted that during the process of hydrolysis the pH of the product increased till 6.5 and such changes are related to cellulase and glucoamylase activity incorporated in the enzyme preparate as well as permeate protein residues hydrolysis. With the increase of syrup total solids, galactose concentration was changed due to galacto-oligosaccharides formation. The degree of sweetness is key factor for the durability of lactose hydrolysis and final syrup concentration.