The impact ionization coefficients for electrons and holes in (100) oriented Ga0.18In0.82As0.39P0.61whose band gap is 1.11 eV have been obtained from photomultiplication measurements on a Cd diffusedp+nabrupt junction having a donor concentration of 2.5×1016cm−3. The electron‐to‐hole ionization coefficient ratio has been found to be about 1.1 in the electric field range 4.0×105–5.0×105V/cm.