The conduction‐band discontinuity ΔEc of AlxGa1−xAs/In0.5Ga0.5P heterojunctions grown by liquid phase epitaxy on GaAs substrate was studied using the capacitance–voltage (C–V) characterization technique. The C–V measurements were made on a series of samples with x ranging from zero to about 0.3. The carrier profiles for the samples with x=0 and x=0.06 give ΔEc values of 90 and 40 meV, respectively, showing the type I (straddling) band line‐up. For x=0.18 and 0.29, the values of ΔEc were 45 and 110 meV, respectively, with the carrier profile characteristic of the type II (staggered) band line‐up. From these results, ΔEc of the heterojunction is found to vanish at about x = 0.12. This agrees well with our previous result determined from the photoluminescence measurements.