A solid solution is a single solid phase containing more than one chemical species distributed within one underlying crystal structure.

In a substitutional solid solution, solute atoms replace host atoms on ordinary lattice sites. In an interstitial solid solution, smaller solute atoms occupy spaces between host atoms.

The distinction is structural rather than simply compositional. Two alloying elements can be completely mixed as one solid phase over some composition range, while another combination separates into two phases even at the same overall solute fraction.

Solute atoms usually differ from the host in atomic size, bonding or local electronic structure. They therefore perturb the surrounding lattice. Those local distortions can interact with dislocations and are one origin of solid-solution strengthening.

Solid solubility is limited when incorporating additional solute into the existing phase becomes less favorable than forming a different phase. Temperature can change that balance, so equilibrium solid solubility often varies strongly with temperature.

A solid solution should not be confused with a mixture of microscopic particles of two phases. In a solid solution, the solute is incorporated atomically into one phase; in a two-phase microstructure, distinct regions have different structures or compositions.