Casing in extreme-temperature sour wells must withstand severe loading that includes large temperature excursions cyclic elastic-plastic deformation and exposure to sour environments. Those conditions lead to synergistic material damage with various degradation mechanisms accelerating one another. Current understanding of synergies between thermo-mechanical loading and sour corrosion-cracking is limited. No industry standard exists for evaluation of tubular materials under such synergistic loading. Full-scale tests of tubular assemblies immersed in sour solutions are sometimes used to assess fitness-for-purpose of specific products but those tests are long and expensive and allow only limited control of impacting variables; and thus are not practical for material evaluation and selection purposes.This paper presents an evaluation methodology that employs reduced-scale specimens and is practical to execute in laboratory conditions. Development of that test methodology has been conducted via a joint industry venture sponsored by 12 thermal well operating companies and 4 OCTG material suppliers. As part of the methodology development customized and standard tests were performed on samples of four OCTG materials. The objective of the test program was to identify dominant failure mechanisms and the parameters that are most relevant to those damage mechanisms; and subsequently determine the level of test complexity required to accurately evaluate the relative material performance under synergistic conditions. The load path of the most complex test in the program the Synergistic Reference Test (SRT) was defined to be as representative as-practical of the worst-case loading and exposure path anticipated by participating thermal well (SAGD and CSS) operators. This load path includes thermomechanical cycles with plastic material deformation in both tension and compression temperature cycling and concurrent intermittent sour exposure. The results were evaluated and compared to a variety of simplified tests with specific test parameters controlled including replacement of concurrent sour exposure with sequential sour exposure. The results comparisons were evaluated based on observed fracture mechanisms (evaluated using SEM) and material ductility loss.While the project is still ongoing the paper presents key results obtained from SRT tests and five simplified test complexities on a range of OCTG materials. The results clearly demonstrate the significance of concurrent post -yield loading and sour exposure reinforcing the need to develop a methodology for material evaluation and selection under these combined loading/exposure conditions. The paper also contains examples of results from simplified tests focused on critical loading components and damage mechanisms. Comparisons between the SRT tests and the simplified tests show clear indications of the required test complexity and the importance of concurrent plasticity and sour exposure.Development of this test methodology will allow an accurate and economic assessment of material performance under combined thermo-mechanical and environmental loads enable a rigorous selection of materials for extreme-service sour wells and thus contribute to enhancing well integrity.