The newly developed axisymmetric tension test is useful in evaluating the performance of geomembranes subjected to out-of-plane stress conditions such as those that can develop due to localized subsidence under waste-containment cover and liner systems. In this paper, the equations necessary to describe the geomembrane stresses and strains during testing are derived, and equations used to calculate geomembrane stresses during axisymmetric field loadings are also presented and compared. Finally, the testing equipment and procedures are investigated.