The initial degradation of II–VI light emitting devices, namely the ⟨100⟩ dark line defect formation in CdZnSe quantum well structures formed during laser or light emitting diode (LED) operation, has been investigated. Optically degraded quantum well structures exhibiting dark line defects were investigated via transmission electron microscopy. The observable dislocation networks have been determined to be conglomerations of dislocation loops confined to the quantum well region having an associated Burgers vectors ofa[100] anda[010]. The discovery and identification of initial dark line defects may prove useful in identifying the defect mechanism in II–VI light emitters. ©1995 American Institute of Physics.