The interface in a binary single quantum-well (SQW) structure of ZnSe/CdSe, where CdSe less than one monolayer is sandwiched by ZnSe layers (submonolayer SQW), is characterized by means of photoluminescence scpectroscopy. The dependence of the energy, linewidth and intensity of excitonic emission from the submonolayer SQWs on the well thickness of CdSe is extensively investigated. The characteristics of the excitonic emission are interpreted in terms of alloy formation at the interface. It is clarified that the ZnCdSe alloy is formed at the interface in ZnSe/CdSe quantum well heterostructure.