Compositions containing sulfuric acid and one or more of certain chalcogen-containing compounds in which the chalcogen compound/H.sub.2 SO.sub.4 molar ratio is below 2 contain the mono-adduct of sulfuric acid which is catalytically active for promoting organic chemical reactions. Suitable chalcogen-containing compounds have the empirical formula ##STR1## wherein X is a chalcogen, each of R.sub.1 and R.sub.2 is independently selected from hydrogen, NR.sub.3 R.sub.4, and NR.sub.5, at least one of R.sub.1 and R.sub.2 is other than hydrogen, each of R.sub.3 and R.sub.4 is hydrogen or a monovalent organic radical, and R.sub.5 is a divalent organic radical. Such compositions are useful for catalyzing organic reactions such as oxidation, oxidative addition, reduction, reductive addition, esterification, transesterification, hydrogenation, isomerication (including racemization of optical isomers), alkylation, polymerization, demetallization of organometallics, nitration, Friedel-Crafts reactions, and hydrolysis. Novel catalysts are disclosed which involve combinations of the chalcogen compound-sulfuric compositions with one or more transition metal halides and/or with one or more surfactants. The surfactant-containing compositions are particularly useful for the treatment of materials containing lipophilic substances. Novel compositions containing a chalcogen compound-sulfuric acid component and one or more organic reactants are also disclosed.