Laboratory assessment of permeability of a groundwater protective barrier. The paper presents results of laboratory assessment of permeability of cement-bentonite self hardening mixtures which have recently been the most often used for construction of cut–off walls mitigating hazard of sanitary landfi lls. Laboratory tests were carried out on reconstituted and natural (undisturbed) material of the barrier. Due to the fact that actual permeability characteristics of reconstituted material are time and density dependent, changes of permeability are described against those variables. Comparison of these data with test results obtained for undisturbed material help to understand the process of changing properties of the barrier in the environment.