Rovibrational bands between closely lying bending vibrational states of normal diacetylene and its bideuterated variant have been recorded in the submillimetre-wave region using a source-modulation spectrometer. The two isotopologues were produced by pyrolysis (1200C) of benzene or benzene-d6 vapour in a flow reactor. Measurements of the ν8 − ν6 band and of its ν9-associated hot band have been extended up to 615 and 420 GHz, respectively. Rotational and vibrational l-type resonance were taken into account in the analysis of the ν8 + ν9 − ν6 − ν9 band; in addition the cubic anharmonic resonance existing between the v8 = v9 = 1 state and the v3 = 1 stretching state has been considered explicitly in the least-squares fits to the observed frequencies. Very accurate values of the band origins and of numerous spectroscopic constants have been determined for both isotopologues.