The driving forces for shape changes and diffusion in nonhydrostatically stressed solids have been distinguished. The driving forces for shape changes are related to differences in the chemical potentials of the components of the solid that are defined at the surfaces, e.g., by their vapor pressures, whereas the driving forces for volume diffusion are gradients in the chemical potentials of the components within the solid. For homogeneously but nonhydrostatically stressed solids that are uniform in state throughout, the former potentials are multivalued and directly measurable (e.g., vapor‐pressure measurements), whereas the latter potentials are unique functions of the state of the solid. In general, no meaningful connection exists between the two sets of potentials. However, for situations in which, for example, the shape of a solid is changing by volume diffusion, the variations in the two sets of potentials become continuously connected.