We have studied the low temperature electrical properties of a range of GaAs/(Al,Ga)As two-dimensional electron gas (2DEG) structures including a series of layers with low carrier densities (ns=(0.2-2)×1011cm-2) and extremely high mobilities; in one of these samples, the mobility at 1.5 K exceeded 107cm2V-1s-1. Because of the very low ionised impurity scattering rates in these layers, a detailed assessment of the acoustic phonon scattering process could be made. From the dependence of mobility on temperature and carrier density, the contributions to the phonon scattering rate from the piezoelectric and deformation potential interactions have been separated and compared with theoretical calculations. These results suggest that, in a 2DEG, the value of the piezoelectric coupling constant, h14, is ∼ 50% greater than in bulk GaAs; this enhancement is similar to that already noted for the deformation potential coupling constant, Ξ. © 1990.