The kinetics of glucose liberation from lactose by means of the β-glactosidase from Aspergillus niger has been studied in a wide range of the main variables. The analysis shows that the kinetic models proposed so far are not adequate. The main finding is that the reaction rate is not linearly correlated to the enzyme concentration—it increase more than proportionally. This nonlinear relationship results because this lactase can distinguish between α-and β-galactose α-Galactose acts as competitive and anticompetitive inhibitor while β-galactose is a competitive one. The competitive inhibition of the α-anomer is approximately 12 times more sever than that of the β-anomer. The kinetics, including a simplified model for the mutarotation of galactose is given for a temperature of 50°C at a pH of 3.5—the most likely conditions for the application of this lactase in acid whey treatment.