It is shown that stresses caused at the interface of a AlxGa1−xAs&sngbnd;GaAs heteroboundary are due to the different thermal expansion coefficients of the two layers involved. These interface stresses are elastic and depend on the crystallographic orientation of the heteroboundary plane. Based on these experimental observations, a planar stress model for double heterostructure (DH) devices is developed which results in stress levels in the order of 108dyn/cm2for typical low‐threshold DH laser structures. It is demonstrated that this type of stress is responsible for the occasional preference of TM modes over the TE modes in DH lasers.