We have studied the influence of chemical ordering on the properties of GaInP/AlGaInP optical waveguide structures and lasers. We have observed mode‐conversion between transverse electric (TE) and transverse magnetic (TM) modes within typically 60 &mgr;m. We show that an ordering‐induced birefringence with the optical axis tilted with respect to the growth direction is responsible for the coupling of the TE and TM modes. A new linear combination of TE and TM modes, the ‘‘super modes,’’ can propagate in the waveguide without change. We show that these super modes govern the polarization of lasers made from ordered material. ©1996 American Institute of Physics.