The occurrence of the piezoelectric effect in a strained zincblende semiconductor layer grown on an (h11) plane results in an internal electric field which can be observed through the optical properties. We report a study of Zn0.85Cd0.15Se/ZnSe quantum wells grown on (211)B GaAs substrates where the quantum confined Stark effect due to the internal field shifts the luminescence to longer wavelengths provided that the quantum well layer is strained. When the well width is greater than the critical thickness the layer begins to relax and the internal field decreases. We have used these measurements to determine that the critical thickness for the onset of strain relaxation of the Zn0.85Cd0.15Se quantum wells grown on (211) oriented substrates is 20 nm. The method will be applicable to materials such as the nitride semiconductors with wurtzite symmetry. © 1998 American Institute of Physics.