Two versions of the acoustic deep-level transient spectroscopy (A-DLTS) technique based on the acoustoelectric effect resulting from the interaction between an acoustic wave and heterostructure interfaces have been used to study deep centres in GaAs/AlGaAs heterostructures. The former uses the high frequency transverse acoustoelectric signal (TAS) arrising from the interaction of surface acoustic wave electric field and free carriers at the heterostructure interfaces. The latter uses an acoustoelectric response signal (ARS) produced by the heterostructure interface when a longitudinal acoustic wave propagates through the heterostructure. Planar GaAs/AlGaAs heterostructure capacitors with electrodes in a field effect transistor configuration were investigated by these versions of the A-DLTS technique. Several deep centres were found and their activation energies and corresponding cross-sections determinated. Both the appearance of some A-DLTS peaks and the shift of practically all peaks of the A-DLTS spectrum with increasing bias voltage can be considered to be the characteristic features of interface states.