A laboratory testing program was performed to evaluate the interface shear strength of a geosynthetic clay liner (GCL) / textured geomembrane interface utilizing two pre-shear inundation methods designed to simulate field conditions. Two commercially-available products were tested, a needlepunched and a stitch-bonded GCL. Oedometer swell tests provided swell data for the two products which were used to design the interface shear testing program. Interface shear tests were performed for 1) GCL samples inundated under a low normal stress for a short time and sheared under a higher normal stress, and 2) GCL samples inundated for a longer period under the design normal stress. The results for the two different GCL materials and the two preshear inundation conditions are compared.