We measured impact ionization rates in AlxGa1−xSb atx=0.06, where the band‐gap energyEgequals the spin‐orbital splitting energy &Dgr;, in an electric field of 1.5×105–3.2×105V/cm. By considering exact field profile in the depletion layer for each sample, the ionization rates of AlxGa1−xSb have been determined to be &agr;=2.35×106 exp(−1.30×106/E) and &bgr;=9.02×105 exp(−9.03×105/E). Although our data have not shown the resonant enhancement of hole ionization rates described by O. Hildebrand, W. Kuebart, and M. Pilkuhn [Appl. Phys. Lett.37, 801 (1980)], exact values of impact ionization rates have been established in practical electric fields required for designing AlxGa1−xSb avalanche photodiodes.