This review covers some of the methods for preparation of sulfonic acids reported recently. Phenols can be sulfonated directly using concentrated aqueous sulfuric acid. The products obtained can be analysed by several methods, e.g., HPLC and ¹H NMR spectroscopy. Dihydroxy and dimesyloxynaphthalenes have been sulfonated by the use of SO₃. The sulfonation process was found to be dependent on the reaction conditions and the stoichiometry. Sulfur trioxide alone can be used as a sulfonating reagent to produce sulfonic acids, but sulfur trioxide-base complexes have been found to be more convenient sulfonating reagents, with the advantage that they are easier to handle. For example, (S)-3-amino-2-oxoazetidine-1-sulfonic acid can be obtained in good yield by the use of a sulfur trioxide-base complex. Sulfur trioxide can be inserted into the metalcarbon bond of various organolithiums and Grignard reagents to produce organic sulfonic acids in high yields.