The impact ionization coefficients for electrons and holes have been measured in In0.2Ga0.8As/GaAs strained‐layer superlattices (SLS’s) for transport perpendicular to the superlattice. The pure electron and hole initiated primary photocurrents were produced inp+nmesa photodiodes having In0.1Ga0.9As alloy photon absorption regions and an SLS active region carrier concentration of 7.4×1015cm−3. The impact ionization coefficient for electrons is found to be larger than for holes with a ratio that varies from 1.8 at 2.7×105V/cm to 1.4 at 3.8×105V/cm.