In coordination chemistry, hydration isomerism is a kind of isomerism that is observed in some solids. Hydration isomers have identical formula but differ with respect to the numbers of water ligands.
One example is the pair .[1] The former has one water of crystallization but the latter does not.
Another example is the pair of titanium(III) chlorides, . The former is violet and the latter, with two molecules of water of crystallization, is green.