© 2022 Hydrogen Energy Publications LLCOne of the promising options in the pursuit of clean and sustainable energy is fuel cell technology. PEM fuel cell stands out among fuel cell technologies due to its high efficiency, compactness, and ability to be used in portable applications. SPEEK (with different sulfonation degrees) and PVA blend membranes, which are thought to create a good synergy for PEM fuel cell, were prepared by using the solution casting method. As a result of the weight loss experiments, it was understood that the membranes dissolve in water regardless of the degree of sulfonation. Thermal crosslinking was carried out to prevent the membrane dissolution in water, which is in continuous contact with water in the fuel cell. Dissolution was mostly prevented by means of the thermal crosslinking process. Changes in the physical, chemical, and mechanical structure of the membrane after thermal crosslinking were comparatively determined by fourier transform infrared (FTIR), thermogravimetric analysis – differential thermal analysis (TGA-DTA), differential scanning calorimetry (DSC), water uptake capacity, swelling property, ion exchange capacity (IEC), dynamic mechanical analysis (DMA), electrochemical impedance analysis and oxidative stability. In addition, single-cell performance tests were performed with the membrane that gave the best results in the characterization analyses. Analysis results showed that thermal crosslinking prevented the dissolution of membranes in water ingreat extent. In addition, it was determined that the thermally crosslinked membranes had a more stable structure.