Experimental results obtained by capacitance‐voltage measurements for an LPE grown GaSb(n)–Ga0.83Al0.17Sb(p) heterojunction are described. They permit the extraction of the discontinuities in the bands ΔEc = 0.14 eV and ΔEv = 0.10 eV. The resulting relation ΔEc = 0.62 ΔEg is different from Dingle's rule but in agreement with other works. Also Ga1−xAlxSb electron affinity (for x = 0.17) is deduced to 3.92 eV.