The band alignment in GaAs:(Al,Ga)As heterostructures has been investigated over the full range of alloy composition. The valence-band discontinuity ΔEv is determined by measuring the activation energy for thermionic emission of holes from p-GaAs over an undoped, square (Al,Ga)As barrier. The use of p-type structures to measure ΔEv circumvents a number of complications involved in the measurement of ΔEc. The parameters required for analysis are determined by different measurements on the same structures and the analysis is performed so that the activation energy, extrapolated to zero bias, yields ΔE v directly. It is found that ΔEv is a linear function of the aluminum mole fraction xAl@B: ΔEv ≅0.55xAl (eV) (0≤xAl≤1). The validity of these data is supported by measurements of ΔEc in the direct band-gap regime, where complementary values of ΔEv and ΔE c add up to the expected band-gap difference. This relationship provides a simple description of the full band alignment in this heterosystem and should prove valuable as a test of the various heterojunction lineup theories. Moreover, these measurements have a number of important consequences, particularly from the viewpoint of heterojunction devices.