This work describes the long-term behaviour of three kinds of geomembranes which are constituted by plasticized poly vinyl chloride (PVC-P), high density polyethylene (HDPE) and terpolymer rubber of ethylene-propylene-dienic monomer (EPDM), used as the waterproofing system of the reservoirs “Los Llanos de Mesa”, “San Isidro” and “El Golfo”, respectively. Characteristics of the three original geomembranes and their behaviour along time are presented. Thicknesses, content and nature of the plasticizers (in PVC-P), tensile properties, dynamic and static puncture, foldability at low temperature, shore hardness, tear resistance and carbon black (in HDPE), joint strength (shear and peeling test) and microscopy, both optical and electronic scanning tests were carried out. Results obtained conclude with a long-term durability of geomembranes, independently of their macromolecular nature. These characteristics were determined by advanced analytical techniques in PVC-P samples, such as Fourier Transform Infrared Spectroscopy (FTIR), Gas Chromatography (GC) and Mass Spectrometry (MS).