The influence that different experimental conditions have on the activity of two commercial beta-galactosidases (Lactozym and Maxilact) has been investigated. Two kinetic models have been proposed to explain the behaviour of the enzymatic activity versus pH, implying the dissociation of one or two protons of the enzyme. The thermal deactivation found for the enzyme beta-galactosidase was fit to a kinetic model. The kinetic parameters have been calculated. The models proposed explain the behaviour of the enzyme with different pH and temperature values, and satisfactorily fit the experimental results obtained in this work as well as the results proposed by other researchers for lactases from different sources. (C) 2003 Elsevier Inc. All rights reserved.