Hypervalent iodine(iii)-derivatives display higher catalytic activity than other aliphatic and aromatic iodine(i)- or bromine(i)-containing substrates for a Knorr-type reaction of N-acetyl hydrazides with acetyl acetone to give N-acyl pyrazoles. The highest activity was observed for dibenziodolium triflate, for which 10 mol% resulted in the generation of N-acyl pyrazole from acyl hydrazide and acetyl acetone typically at 50 °C for 3.5-6 h with up to 99% isolated yields. 1H NMR titration data and DFT calculations indicate that the catalytic activity of the iodine(iii) is caused by the binding with a ketone. This journal is