<p>Polymers with ion charges anchored to their structure, known as ionomers, are slightly modified materials with notable applications for electrochemical processes. However, the characterization and quantification of those sulfonic groups grafted to the polymer is uncommon, which can lead to inaccurate data regarding their performance. A styrene-co-butyl acrylate copolymer (83:17) was synthesized and further sulfonated with concentrated sulfuric acid. Analytical techniques such as proton Nuclear Magnetic Resonance (¹H NMR) and thermogravimetric analysis (TGA) were employed to characterize ionomers and used to calculate the degree of sulfonation (DS). Membranes were prepared by casting from the sulfonated copolymer, using two different solvent systems; then, the ion exchange capacity (IEC) was evaluated to be able to calculate DS values. For the ionomer, a DS of 35.2% was obtained by ¹H NMR and 38.4% by TGA, while from membranes IEC results the DS ranged from 4.9% to 28%, depending on the solvent system used for membrane preparation. The DS values in the ionomer are consistent between both analytical techniques, as each quantifies sulfonic groups from a different perspective: NMR considers the solubility of the material and thus reflects only sulfonic groups, whereas TGA accounts for the total mass loss, potentially including both sulfonic and sulfone (Ar–SO₂–Ar) groups. By contrast, the widely used IEC method must be interpreted with caution, since it is more closely related to the structural arrangement of the membranes and the accessibility of ionic groups for exchange, which are strongly influenced by the preparation method. Overall, the three techniques provide complementary estimations of DS, whether in the ionomer or membrane form, and should therefore be considered as mutually supportive approaches.</p> Graphical abstract <p></p>