The conduction‐band discontinuity &Dgr;Ecand interface charge density &sgr; have been studied for GaAs/In0.5(Ga1−xAlx)0.5P heterojunctions, prepared by metalorganic chemical vapor deposition. The dependences of &Dgr;Ecand &sgr; on Al compositionxwere investigated forxfrom 0 to 1. The In0.5Al0.5P/ In0.5Ga0.5P heterojunction was also examined. The results suggest that the valence‐band discontinuity &Dgr;Evfor GaAs/In0.5(Ga1−xAlx)0.5P is a linear function ofxand is larger than &Dgr;Ec, being in reasonable agreement with results on the InAlP/InGaP heterojunction. The &sgr; values for GaAs/InGaAlP were found to be one order of magnitude larger than those for GaAs/AlGaAs and InAlP/InGaP heterojunctions.