The ‘Laguna de Barlovento’ reservoir was, at the time of its construction, one of the most important European hydraulic projects because of its high capacity and challenging location. The engineers decided to waterproof it with a plasticized polyvinyl chloride (PVC-P) geomembrane. PVC-P geomembrane is exposed to atmospheric agents without any external protection. This paper documents the initial characteristics of the geomembrane and its performance since its installation in 1991 until 2014. The results presented here, include quantification of geomembrane thickness, content and nature of plasticizers, tensile properties, foldability at low temperatures, dynamic impact resistance, puncture resistance, welding strength and welding peeling resistance, as well as the use of optical and scanning electron microscopy. To identify the plasticizers nature, had been used analytical techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Gas Chromatography (GC) and Mass Spectrometry (MS).