To study origins of alternating current thin‐film electroluminescent display (AC‐TFELD) degradation phenomena, we have fabricated two different types of AC‐TFELD with multilayered insulators. For an accelerated aging, high‐frequency voltages were applied and brightness versus operating time characteristics were measured. Surface morphology (SEM), Auger electron spectroscopy (AES) in‐depth profiles, and AES spectra were examined for the aged devices with and without Si3N4 interlayers. From the obtained results, we proposed that the formation of zinc‐oxy‐sulfides in ZnS:Mn‐insulator at the ITO side may play a significant role in the aging phenomena and degradation of brightness (B)‐operating time (T) characteristics for our devices. Therefore, we suggest that the dielectric interlayer of stable stochiometry must be inserted in ZnS:Mn‐insulator at the In2O3‐SnO2(ITO) side to improve the aging characteristics.