An analytically tractable theory is developed and solutions are presented describing the simultaneous action of three processes occurring during the phase transformation of a single phase fluid into a dispersion of partially or immiscible liquids. These processes are nucleation, diffusional growth and sedimentation due to the action of gravity. A general partial differential equation describing the transformation is developed and solved. Some special cases like a constant or time dependent nucleation rate are treated explicitly. They are compared with experiments performed with immiscible aluminumbismuth alloys. The agreement between theoretical prediction and experimental results is satisfactory. * Especially in organic liquids the position of the coexistence line in the phase diagram can depend appreciably on pressure such that the system is single phase at a high pressure --order 1-10 MPa --and two phase at normal pressure. If such a fluid is heated under the action of a high pressure and then only the pressure is released the system is nearly instantaneously pushed into the two phase regime of the phase diagram at nearly constant temperature.