In catalytic applications of surface-modified mesoporous silica materials, distinguishing and quantifying different types of functional groups on the surface is crucial for enabling accurate evaluation of catalytic activity and possible cooperativity among mixed functional groups. We investigated sulfonic acid groups supported on SBA-15 mesoporous silica as a promising organic–inorganic hybrid solid acid catalyst and developed a robust technique for sulfur quantification and speciation after oxidation of grafted thiol groups to sulfonic acid. We combined SH-selective titration (Ellman's test), acid–base titration and ICP-OES analysis to quantitatively monitor S speciation at each step of the preparation of these catalysts. Ellman's test confirmed the successful grafting of SH groups on SBA-15, which is in good agreement with ICP-OES total S determination (∼0.8 mmol g_cat⁻¹). These SH groups were converted in a subsequent 30% H₂O₂ treatment to SO₃H groups, while significant leaching of S during oxidation limited the yield of grafted SO₃H (∼0.3 mmol g_cat⁻¹) in the final catalyst. The influence of grafting and oxidation conditions on S speciation was investigated, and the presence of partially oxidized S on the surface was quantified. Despite the loss of a fraction of grafted S, the prepared sulfonic acid-functionalized SBA-15 catalysts exhibited comparable catalytic activity per H⁺ with homogeneous sulfonic acid catalysts in esterification reactions. Re-grafting of SH groups after oxidation was shown to be effective in achieving the coexistence of SH and SO₃H groups on the surface, and a catalytic study revealed no cooperativity between SH and SO₃H for the esterification reaction.