Geosynthetic clay liners (GCL) are factory- manufactured hydraulic barriers consisting of a mineral (bentonite) and geosynthetic component (geotextile or geomembrane). Over the last several decades, their application has increased intensively in various engineering projects for environmental protection purposes, including landfills. In Croatia, there is also a significant number of landfills with GCL incorporated as an integral part of protective systems, most often as the landfill final cover. Since protective systems have to fulfil their designated function over a longer period of time, usually, laboratory tests of the GCL durability and landfill monitoring are being implemented in order to identify long-term functionality of GCL. This study presents laboratory tests carried out on samples of bentonite clay and GCL that were installed at one real landfill near Zagreb between three and nine years ago. The sampling was carried out at 23 locations in the landfill final cover. Following analyses were performed on collected samples, as well as on reference sample (manufactured bentonite in its original state): pH, electrical conductivity, moisture, colour, index properties (free swell index, water absorption capacity by Enslin-Neff test, fluid loss). Mineralogical and chemical analyses were carried out on 13 samples. Mineralogical analyses of bentonite included detailed XRD clay analysis and cation exchange capacity using the Cu-trien method. Hydraulic conductivity was tested on ten GCL samples. Mineralogical analyses proved that montmorillonite is predominate mineral followed by quartz, K-feldspar, plagioclase, goethite, anatase and micaceous material. CEC ranged from 51.96 meq/100g to 75.47 meq/100g. Index properties for all samples are summarized in table 1. It can be concluded that the properties of all samples were deteriorated with time in comparison to the reference sample test results. Moreover, none of them fulfills the required values according to some recommendations nowadays. However, final conclusions about the influence of the cover layer thickness and the time passed since the installation date cannot be precisely divided yet. Therefore, some additional investigations will be carried out.