The decrease in stress crack resistance (SCR) of geomembranes (GMBs) due to physical ageing is examined. It is shown that the SCR of a 1.5 mm high density polyethylene (HDPE) GMB immersed in municipal solid waste (MSW) leachate, deionized (DI) water, and air experienced a decrease to an equilibrium value, denoted herein as SCR, in a few months and then did not change for the remainder of a 116-month test. The same GMB was also immersed in basic mining pregnant leach solution (PLS) and brine and a similar decrease in SCR to SCR was observed in the first three months. For this GMB, SCR was about 50% of the initial SCR value (i.e., SCR ∼ 0.5·SCR) for all incubation fluids examined. It is then shown that this behaviour is not unique. Seven other high density polyethylene (HDPE) and one blended polyolefin (BPO) GMBs are shown to experience a decrease in SCR to 0.17·SCR ≤ SCR ≤ 0.56·SCR without any evidence of oxidative degradation. The magnitude of SCR is shown to be product specific. In contrast, the one linear low density polyethylene (LLDPE) examined exhibited no such physical ageing. The implications for designing with GMBs are discussed.