Abstract Hydrobromic acid was found to be a unique catalyst in C [BOND] C bond-forming reactions with ketene dithioacetals. Distinctly different from other acids (including Lewis and Brønsted acids), the remarkable catalytic performance of hydrobromic acid in catalytic amounts was observed in the “acid”-catalyzed reactions of readily available functionalized ketene dithioacetals 1 with various electrophiles. Under the catalysis of 0.1 equivalents of ...