This paper describes the experimental study, by solutal simulation, of a thermal plume in a confined stratified environment, a situation encountered in displacement ventilation systems. The criteria enabling similarity to be established between the thermal plume in air and the solutal plume in a hydraulic model are discussed. Density stratification is detected by a planar laser-induced fluorescence (PLIF) technique. Criteria for defining the interface height and thickness are determined After validation of these criteria in the fully developed region of the plume in a confined stratified environment, a formulation of the stratification height in the region close to the source has been established.