The service life of high-density polyethylene (HDPE) geomembranes is mainly controlled by the slow loss of antioxidants and stabilisers. The problem is conventional immersion testing typically requires a minimum of 3 to 6 months (and sometimes years) to deplete the additives of high-performance geomembranes to values which are meaningful for the purpose of estimating the service life via the Arrhenius model. This paper presents the results of more than 10 different modern geomembranes subjected to conventional immersion testing and shows there is a large variation in performance of each geomembrane. An additional problem is manufacturers do not disclose the mix of proprietary blends of antioxidants and stabilisers, and for each category of additive there exists 4 to 5 options each varying in functionality, performance, acid-tolerance, alkali tolerance, extractability and cost. What you end up with in terms of durability is a lottery. A way to overcome these issues is by reverse engineering the stabiliser package and de-formulating the additive mixture. The additive deformulation combined with knowledge of the acid and alkali tolerance of each additive allows one to predict the performance of the geomembrane for a given service environment. This paper shows the additive analysis of 6 common HDPE geomembranes and correlates the mix of additives present to the service life as predicted by Thin Film Accelerated Immersion (TFAI) testing. The process of TFAI testing involves compression moulding of geomembrane samples to uniformly thin films and then immersing in liquor with continuous circulation at 85 degrees Celsius to increase the rate of additive leaching. The continuous fluid flow prevents formation of stagnant boundary layers that otherwise reduce additive migration. The thin film increases the surface area to volume ratio so that the diffusion path for the additives is minimised thus increasing the rate of additive loss. It also exposes the geomembrane to a higher surface area of liquor contact so acid-catalysed reactions are accelerated hence increasing additive consumption and additive neutralisation/deactivation. Furthermore, the continuous sweeping flow of liquor over the geomembrane sample increases the concentration gradient creating an enhanced driving force for additive migration, leaching and depletion. The above accelerating factors allow geomembrane samples to be assessed for long-term durability in as little as 2 weeks. The use of TFAI testing can give a 10 fold acceleration in additive depletion compared to conventional immersion testing for 2.5 mm thick geomembranes. Furthermore a sweeping flow of liquor is expected to give an approximately 2 fold increase in additive depletion rates. Therefore, the cumulative advantages of the TFAI testing gives up to an approximate 20 fold acceleration in additive depletion rates compared to conventional immersion testing. Geomembrane durability assessments can be performed faster and cheaper.