Inorganic acid catalysis is acceleration of a reaction by an inorganic Brønsted acid, such as sulfuric, hydrochloric, phosphoric, perchloric, or a heteropolyacid, with regeneration of the acid after product formation. The central elementary step is reversible proton transfer from the acid or its solvated form to a substrate. Protonation can increase carbonyl electrophilicity, convert a hydroxy group into a better leaving group, or generate a carbocationic intermediate; subsequent bond formation or cleavage and deprotonation complete turnover. Silica–sulfuric acid and alumina–sulfuric acid retain Brønsted acidity while operating as supported heterogeneous catalysts ^[1]. Sulfonic-acid-functionalized mesoporous silica represents a related solid-acid class used in organic reactions ^[2]. The term is defined by the inorganic proton donor, not simply by low pH. It therefore excludes Lewis-acid catalysis by electron-pair-accepting metal centers when no proton transfer is involved, and it differs from organic Brønsted-acid catalysis by carboxylic, sulfonic, or chiral phosphoric acids.