Ostwald ripening is an observed phenomena in solid solutions which describes the evolution of an inhomogenous structure over time. When a phase precipitates out of a solid, energetic factors will drive some precipitates to grow, drawing from the others, which shrink. If this process continues, eventually fewer and larger crystals form inside the solid that have smaller and smaller surface-to-volume ratios compared to the smaller particles, thus reducing the energy of the entire system.

The phenomenon was first described by Wilhelm Ostwald in 1896.

In chemistry, the term refers to the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones. In the process, many small crystals formed initially slowly disappear, except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as fuel for the growth of bigger crystals.

This spontaneous process occurs because larger particles are more energetically favored than smaller particles. While the formation of many small particles is kinetically favored, (i.e. they nucleate more easily) large particles are thermodynamically favored. This is because small particles have a larger surface area to volume ratio than large particles and are consequently easier to produce. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. Large particles, with their greater volume to surface area ratio, therefore represent a lower energy state. Hence, many small particles will attain a lower energy state if transformed into large particles and this is what we see in Ostwald ripening.