This paper provides to show the performance of synthetic geomembranes based on high density poliethylene (HDPE). Results along seven years are presented. The samples of geomembranes came from several Spanish reservoirs: Betíes-II, Buen Paso, Cabezo de los Leones, El Palmar, El Reclot, La Mericana, La Rambla de Carcáuz, La Rueda, Los Cabezos, Pilar de la Horadada, San Isidro, San Lorenzo, San Xulián de Vigo and El Saltadero (experimental field)). Important characteristics of high density polyethylene were evaluated in function of time once applied in reservoirs and they were the following ones: tensile properties, tear resistance, impact resistance, static perforation, low temperature folding, joint strength, joint peel resistance, carbon black: content and dispersion, optic microscopy and electron microscopy “scanner”.