Miel I is a 188 m high RCC dam in Colombia, at elevation 454 m a.s.l. The RCC mix has a cement content of 85 to 160 kg/m3. To meet contractual schedule, the original design of an upstream face made of slip formed reinforced concrete was changed to a Carpi drained exposed composite Polyvinylchloride geomembrane (= PVC geocomposite) system, placed on a 0.4 m thick zone of RCC material enriched with cement grout and vibrated. Placement of the geocomposite system in horizontal stages allowed starting of dam impoundment while RCC placement was still ongoing at higher elevation. The geocomposite system was completed in 2002, before finishing of complementary concrete works. ISAGEN, the owner of the dam, constantly monitors the dam’s and geocomposite system’s performance. The average leakage of the dam registered since the first impoundment of the reservoir has been of 2 l/s for the geocomposite’s drainage system, and of 25 l/s from the abutments. Such levels are below the design parameters that defined the maximum acceptable leakage through the geocomposite in 9.7 l/s and the maximum acceptable leakage through the abutments in 30 l/s. The measured leakage has never surpassed the historical values (with stable lecture through 11 years), or overcome the maximum allowed drainage design values defined during the design stage. After 11 years of good performance and operation, in summer 2013 ISAGEN decided to have a comprehensive monitoring of the general conditions and the weathering behaviour of the waterproofing geocomposite at Miel I. The objectives of the investigation were: assessing the reason of small cuts that had been detected and repaired (in total 15 in 11 years on a 31,000 square meters surface), assessing the conditions of the PVC geocomposite, ascertaining at which extent its characteristics had changed over service, and evaluating if the weathering behaviour was in accordance with the expected durability. Investigation started with inspection at the upstream face and in the drainage galleries, followed by sampling of the exposed PVC geocomposites. Testing in an independent laboratory allowed comparing the physical and chemical properties of the aged geocomposite sample collected at the dam with the physical and chemical properties of the same geocomposite as manufactured. Evaluation of test results, based also on most recent research on geomembrane weathering mechanisms, completed the investigation. Results confirmed that the behaviour of the PVC geocomposite is extremely good and fully in line with expectations. The paper, after a brief outline of the geocomposite system installed at the dam, will describe all steps of the investigation, and detail the test results and the conclusions drawn.