Alkaline stability of anion exchange membranes (AEMs) is an essential requirement for the practical application of alkaline membrane fuel cells. In this study, we investigate the alkaline stability of Diels-Alder polyphenylenes (DAPPs) under various stability testing conditions. Structural analysis and membrane properties of the DAPPs indicated that different chemical structural changes of quaternized DAPPs occur depending on test conditions. Benzyltrimethylammonium-functionalized DAPPs degraded rapidly via nucleophilic benzyl substitution under a relatively mild condition at 80 degrees C with 4 M NaOH, whereas DAPPs with hexyltrimethylammonium (HTMA DAPP) exhibited much better alkaline stability under the same condition. While only cross-linking of unreacted alkyl bromides of HTMA DAPP occurred at 80 degrees C, we observed the degradation of the cations via beta elimination after long-term testing. At higher temperature or reduced relative humidity (RI-1) conditions, for example, 160 degrees C with 8 M NaOH or RH 10% at 100 degrees C, the rapid degradation of the cation in HTMA DAPP occurred via nucleophilic methyl substitution. These results suggest that accelerated stress tests may not be a quick and straightforward alternative to prolonged alkaline stability tests of AEMs.