A model of the diffusive interface in double-diffusive convection at high Rayleigh number is proposed. The interface is assumed to have a double structure: two marginally stable boundary layers from which blobs or thermals arise on the outer edges of the interface, separated by a diffusive core across which all transport takes place by molecular diffusion. The model is time-independent and comparison is made with unsteady ‘run-down’ experiments on the assumption that the experiments run down through a sequence of equilibrium states each of which can be considered separately. The model predicts a constant ratio of the buoyancy fluxes of the two components at a value equal to the square root of the ratio of their molecular diffusivities, and individual fluxes in reasonable agreement with the available experimental data. Some time-dependent features of the model are also examined.

The transport properties of a diffusive interface with diffusivity ratio $\kappa_S/\kappa_T = {\textstyle\frac{1}{3}}$ have been measured, using salt and sugar as the diffusing components. The flux ratio is constant and equal to (κS/κT)½. The normalized salt flux is related to the density anomaly ratio Rρ = βΔS/αΔT by the power law F*T = 2·59Rρ−12.6 over four decades. Optical measurements show that the vertical gradients of concentration of salt and sugar within the interface are those required if molecular diffusion is to account for the whole flux of each component.

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The phenomenon of salt fingers has been investigated optically to determine the geometry of the cells as seen from above. When the fingers are short, the flow appears to be highly turbulent, though a dominant scale is evident. When the fingers are longer, a cellular structure is clear. This structure changes only slowly, apparently in response to disturbances in the convecting layers which bound the fingers above and below, and becomes more nearly stationary as the fingers grow. Cell boundaries show a strong tendency to intersect at right angles, which favours the emergence of cells with a square horizontal section. As the fingers get longer the cell width increases, but more slowly than the length.