Summary A series of sulfonated polymers from poly(ether ether ketone) (PEEK) and poly(ether ether sulfone) (PEES) were obtained using acetyl sulfate as mild sulfonating agent. The thermoplastic polymers PEEK and PEES were synthesized by polycondensation reactions and sulfonated by electrophilic substitutions of the most activated positions in the aromatic polymer structure. The sulfonation reaction was performed between 40 °C to 60 °C varying the reaction time from 3 h to 12 h to obtain different Degrees of Sulfonation (DS). The modification of these polymers was corroborated by spectroscopic techniques and the thermal properties were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Finally, ABA triblock copolymers, such as PMMA‐PEEK‐PMMA and PMMA‐PEES‐PMMA, were sulfonated under soft reactions conditions, and the modification of middle block of each copolymer was confirmed by different techniques.