Initially homogeneous suspensions of colloidal particles often develop patterns during sedimentation. Commonly, the concentration profile of the particles evolves into a “staircase”: layers of nearly constant concentration, separated by sharp boundaries between successive layers, with the concentration of each successive layer increasing with depth. Siano  has demonstrated experimentally that uphill diffusion, diffusion against the concentration gradient, occurs during this pattern formation. Thus, these patterns appear to be the result of spinodal decomposition. We find that these staircase patterns cannot be explained by the classical spinodal decomposition theory of Cahn and Hilliard, but that they can be explained if the linear gradient-energy term of Tiller, Pound, and Hirth is added to the free energy. Such a term plays a central role in the faceting of crystals. In the present application we believe that the physical origin of this extra term may be the Rayleigh-Taylor instability.