The band alignment in GaAs:(Al,Ga)As heterostructures has been investigated over the full range of alloy composition. The valence‐band discontinuity &Dgr;Evis determined by measuring the activation energy for thermionic emission of holes fromp‐GaAs over an undoped, square (Al,Ga)As barrier. The use ofp‐type structures to measure &Dgr;Evcircumvents a number of complications involved in the measurement of &Dgr;Ec. 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 &Dgr;Evdirectly. It is found that &Dgr;Evis a linear function of the aluminum mole fractionxAl@B:&Dgr;Ev&bartil;0.55xAl(eV) (0≤xAl≤1). The validity of these data is supported by measurements of &Dgr;Ecin the direct band‐gap regime, where complementary values of &Dgr;Evand &Dgr;Ecadd 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.