The relationship between the magnitude of the piezoelectric field and the degree of lattice constant mismatch is investigated via low‐temperature photoluminescence measurements of the quantum‐confined Stark effect for a series of (111)B Al0.15Ga0.85As‐InyGa1−yAs pseudomorphic quantum well heterostructures. The experimental strain‐induced electric field values agree well with theoretical calculations for indium mole fractions in the range 0.037≤y≤0.09. In addition, an anomalous saturation of the photoluminescence transition energy is observed at values of applied voltage greater than that required to nullify the piezoelectric field, despite the indication from separate electroreflectance measurements that the net electric field within the quantum well reverses polarity under similar electrical biasing conditions.