In industrial electrodialysis (ED) processes, to ensure a stable and reliable operation for a long period, alkaline cleaning processes, such as cleaning in place (CIP), are often used to suppress fouling of anion exchange membranes (AEMs). However, alkali CIP causes a reduction in the AEM performance, especially for AEMs containing polyvinyl chloride (PVC) as backing cloth and/or binder. To clarify the relationship between alkali attack and performance reduction of AEMs quantitatively, an alkali immersion test was conducted using a commercially available AEM containing PVC, with different immersion times, at various temperatures and NaOH concentrations, to obtain test pieces. Electrochemical and mechanical measurements of the test pieces revealed a reduction in Young's modulus and subsequent increase in water content of the AEMs and resulted in a reduction of the electrical resistance and proton rejection of the AEMs. ATR-FTIR and XRF analyses support that dehydrochlorination of PVC and production of polyene in the AEMs by alkali resulting in such performance reductions. Visible light (VIS) absorption and microscopic observation of the test pieces showed that polyene was produced in the backing cloth and binder. Moreover, using the VIS absorbance data, a prediction method for performance reduction in an AEM was proposed.