In dichloromethane at 0°C, the acyl fluorides 3 insert sulfur trioxide to form the thermolabile arylcarbonic fluorosulfonic anhydrides 4 in good yield. Acylation of non-activated arenes with the anhydrides 4 is impossible since they decompose upon warming into the constituent compounds; with the SO3 thus liberated, diarylsulfones 6 are formed