There is a strong belief that sulfonated polyether ether ketone (SPEEK) can be used as an alternative to Nafion membrane in PEMFC applications due to its properties that can vary depending on sulfonation degree (DS). In this study, the temperature of the sulfonation reaction was kept constant at 50 degrees C, and SPEEK membranes were synthesized by altering the sulfonation time. The obtained membranes were characterized by proton nuclear magnetic resonance (H-NMR), Fourier transform infrared spectroscopy (FTIR), thermogravimetric and differential thermal analysis (TGA-DTA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), electrochemical impedance spectroscopy, water uptake capacity, swelling property, and ion exchange capacity tests (IEC). The kinetics of the sulfonation reaction was examined with experimental data and theoretically, the sulfonation degrees corresponding to the sulfonation time were determined. It has been ascertained that at least 4 h are required for the PEEK pellets to dissolve in sulfuric acid, while lower times are not sufficient for the sulfonation reaction. It has been determined that synthesized membranes have good thermal properties and the crystallization temperatures are independent of the sulfonation degree. Although the presence of sulfonic acid groups in the structure positively affected the water uptake capacity, ion exchange capacity, and hydration degree, deterioration was observed in the mechanical properties. High proton conductivity was obtained with the as-prepared membranes, and it was observed that activation energies were varied between 8.08 and 12.34 kJ/mol. The proton conduction law and transport mechanism have been determined to conform to the Arrhenius law and Grotthuss mechanism, respectively.