[en] The amount of mixing at the molecular level at various stages during the growth of Rayleigh-Taylor instability is investigated in a series of laboratory experiments. Instability is produced at an interface between two miscable liquids, brine and fresh water, with zero surface tension. The brine, density ρ + Δρ with Δρ > 0, is placed on top of a layer of fresh water, density ρ, and separated from it by a horizontal barrier. Both liquid layers are initially at rest, and the instability is initiated by removing the barrier horizontally. Measurements consist of visual observations of the flow using fluorescent dye and shadow-graphs. Quantitative measurements of the amount of mixing are obtained from analysis of dye intensities profiles during the growth of the instability. These measurements give an integrated value (across the width of the mixing region) of the amount of mixing. A conductivity probe has been used to make point measurements of the salinity. These have been correlated with visual observations of relate changes in salinity with specific mixing processes