The microstructural degradation of a lattice‐matched Ga0.28In0.72As0.61P0.39/InP heterointerface during Zn diffusion has been investigated using high resolution transmission electron microscopy and Auger electron spectroscopy. The diffusion‐induced intermixing of In and Ga across the GaInAsP/InP interface causes tensile stress in the Ga‐mixed InP side and compressive stress in the In‐mixed GaInAsP side. The effect of the localized interfacial stress on the nucleation of misfit dislocations and on the strain accommodation behaviors thereof are clearly revealed throughout the intermixed region, reaching several thousand angstroms on each side of the interface. The interfacial strain is relaxed by generation of paired dislocations with antiparallel Burgers vectors initiating from the intermixed GaInAsP/GaInP interface. The dislocation morphologies reveal striking contrasts across the intermixed interface: stacking faults in the tensile layer and perfect dislocation tangles in the compressive layer. The dislocation lines are concentrated at the GaInAsP/GaInP interface and along the misfit boundaries in the forefront areas of the intermixed region. A model is proposed to explain the strain relaxation behavior in the intermixed region using the mechanism of homogeneous nucleation and splitting of the paired dislocations from the intermixed interface. Also observed in a limited region on the GaInP side is the precipitation of a Zn3P2phase. The Zn3P2precipitates grow to form epitaxial layers to a certain depth of the intermixed GaxIn1−xP layer, where the Zn3P2crystal lattice coherently matches with the matrix crystal lattice. The precipitation reaction of Zn3P2is explained using the kickout mechanism.