The quantum confined Stark effect has been studied in InGaAsP/InP rib waveguide structures grown by gas source molecular beam epitaxy. Using 100‐A˚‐wide wells of InGaAs, a room‐temperature exciton shift of about 250 A˚ has been observed for a bias voltage of 6 V. At a wavelength of 1.64 &mgr;m a modulation depth of 35% has been achieved at a frequency of 500 MHz. We show that efficient modulation at shorter wavelength cannot be obtained in structures with thinner ternary wells. Instead, we propose and demonstrate InGaAsP modulators operating at a wavelength as short as 1.3 &mgr;m.