The influence of the reactant ratio and amount of dehydrating agent on the yield of 4,6-diphenyl-1,2-oxathiin 2,2-dioxide in the reaction of phenylacetylene with sulfuric acid was studied. The product can decompose in aqueous acidic medium to give acetophenone. The highest yield of 4,6-diphenyl-1,2-oxathiin 2,2-dioxide was obtained in the reaction of phenylacetylene with the SO3-dioxane complex or a solution of SO3 in chloroform-dioxane. Diphenylacetylene with SO3 does not form the corresponding β-sultone but undergoes oxidation to benzyl. Diphenylbutadiyne does not react with the SO3-dioxane complex.