Geomembrane-geotextile interface shear strength parameters were measured using inclined board (tilting table), standard size (60 mm x 60 mm) and large-scale (300 mm x 300 mm) direct shear box tests. Smooth and rough HDPE, PVC geomembranes and nonwoven needle-punched geotextiles were used in the experiments. Inclined board tests were conducted under a normal stress range of about 5-50 kPa on variable size interface areas. In direct shear tests the applied normal stresses were about 25-300 kPa and 110-400 kPa in the small and large size tests, respectively, The interface friction angle and adhesion intercept values obtained by fitting a straight line through the plots of interface shear strength versus the normal stress were compared for different tests. It has been found that inclined board test results produce envelopes with relatively small adhesion values for both smooth and rough HDPE geomembrane-geotextile interfaces. The size of the interface does not appear to be a significant factor. For smooth geomembranes direct shear and inclined board tests give comparable interface friction angle and adhesion values. For rough geomembranes, however, the direct sheer adhesion values are markedly higher and friction angle values are a few degrees higher when compared to the inclined board test results. (C) 2001 Elsevier Science Ltd. All rights reserved.