The conduction band discontinuities in Ga0.5In0.5P‐AlxGa0.5−xIn0.5P heterojunctions, lattice matched to GaAs and grown by molecular beam epitaxy, were measured by internal photoemission techniques at room temperature over the whole compositional range. The discontinuity is found to vary linearly inxas (0.59x)eV forx≤0.30 and as (−0.18x+0.23)eV forx≳0.30, whereas the inferred valence band discontinuity (band‐gap difference minus the conduction band discontinuity) varies as (0.61x)eV. The direct–indirect gap crossover composition in AlxGa0.5−xIn0.5P is found to be close tox≊0.3. ©1995 American Institute of Physics.