In this study, an ex-situ method is used to investigate the mechanical degradation of sulfonated poly (ether ether ketone) membranes. The mechanical degradation is carried out by hygrothermal cycles test in different cycles as 1 s t to 10 t h cycles as a function of temperature and relative humidity for 700 min. The water uptake, dimensional stability, mechanical properties, and hydrogen crossover through specimens of degraded membranes are investigated after different cycle numbers. Additionally, morphological and structural characterizations included field emission scanning electron microscopy, atomic force microscopy, and X-ray diffraction are employed for understanding of mechanical degradation effects on SPEEK membranes properties. Results suggest that SPEEK membrane due to simultaneous creep and exposure to elevated temperatures and various humidity levels encounters microstructural rearrangement. The mechanical results show the ultimate tensile strengths and Young modules of SPEEK membranes increase and SPEEICs' toughness drops by approximately 80% at the end of the experiment and they act more brittle in the tensile test. Moreover, the AFM results show preexistent dead-end hydrophilic domains join each other to form more hydrophilic morphology within SPEEK and accordingly, water uptake increases. Furthermore, hydrogen crossover rate increases due to thinning and micro-flaws formation in the thickness direction and morphology evolution.