We demonstrate for the first time that large quantum‐confined Stark effect can be obtained in high quality InGaAsP/InP quantum wellp‐i‐nheterostructures. The compositional flexibility of this material system is particularly suited for quantum well device applications in optical communications systems operating between 1.55 and 1.3 &mgr;m. We measure the magnitude of the shift of the ground‐state exciton transition with applied electric field and find that it is significantly enhanced over ternary wells. We have also determined the electro‐optic intensity and phase modulation response in these structures.