DFT and CCSD(T) frequencies are compared with experiment for ortho-benzyne (C6H4). Four bands are found to be in disagreement with experiment at the DFT level. Surprisingly the CCSD(T) method only brings the triple bond stretch into agreement with experiment, but leaves a sizable difference with experiment for the other three bands. The results for three isotopologues suggests that all of the differences cannot be attributed to resonances. Additional experimental work on ortho-benzyne appears warranted. Published by Elsevier B.V.