El Boquerón reservoir (Canary Islands, northern hemisphere) is waterproofed with an ethylene-propylene-diene terpolymer (EPDM) geomembrane (GM). To evaluate the durability of EPDM GMs in hydraulic applications, the characteristics of the EPDM GM (resin and additives) installed in the El Boquerón reservoir were followed for over 21 years. The results of this study are presented herein and show that, based on measurements of specimens from the north and south slopes of the reservoir, the orientation of the GM influences its ageing, with the north slope in the coronation or crest area aging the most rapidly. Over the years, the El Boquerón EPDM GM oxidized, which may be the main cause of crosslinking in the EPDM. In turn, crosslinking could be the mechanism behind some of changes observed in the EPDM-GM characteristics over time. In addition, some of the mechanical characteristics of the EPDM GM were influenced by the additives in the formulation; in particular, the loss and oxidation of paraffinic oils. However, despite the age-induced evolution of the mechanical and physicochemical characteristics of the GM, the El Boquerón EPDM GM, after 21 years of exposure, still satisfies the minimum requirements established by French standards for virgin GMs.