Buoyancy forces in nonuniform dispersions are considered. Using a thermodynamic approach it is shown that Archimedes principle, which holds in cases of pure fluids and uniform dispersions, is incomplete in the case of nonuniform dispersions. For nonuniform dispersions the Archimedes force density must be augmented by two diffusion‐type buoyancy force densities which are dependent on the expectation of particle size and density. The generalized buoyancy force consists of three vectors that may have different directions. It is shown that the effect of buoyancy diffusion is pronounced at diffusion fronts, being enhanced by aggregative motion of particles. Finally, the effect of buoyancy diffusion is expected to prevail over Brownian diffusion in the post‐micron particle size range.