In this work, the band offsets at theGa0.5In0.5P/AlxGa0.5−xIn0.5Pheterojunction lattice matched to (001) GaAs was calculated over the whole range of aluminum composition fromx=0.0to 0.5 using the first-principles pseudopotential method with virtual crystal approximation. The valence band offset, VBO, varies withxasVBO=0.433x eV,while the inferred conduction band offset CBO at &Ggr; minimum (band-gap difference minus the valence band offset) varies inxasCBO&Ggr;=0.787x eV.Our results are in good agreement with the experimental data. ©1998 American Institute of Physics.