This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants.