Isostructural chemical compounds have similar chemical structures. "Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous substances crystallise in the same space group and have the same unit cell dimensions.^[1] The IUCR definition^[2] used by crystallographers is:

Two crystals are said to be isostructural, if they have the same structure, but not necessarily the same cell dimensions nor the same chemical composition, and with a 'comparable' variability in the atomic coordinates to that of the cell dimensions and chemical composition. For instance, calcite CaCO₃, sodium nitrate NaNO₃ and iron borate FeBO₃ are isostructural. One also speaks of isostructural series, or of isostructural polymorphs or isostructural phase transitions. The term isotypic is synonymous with isostructural.

Examples include:

• I-Gold(I) bromide is isostructural with gold(I) chloride
• Borazine is isostructural with benzene
• Indium(I) bromide is isostructural with β-thallium(I) iodide and has a distorted rock salt structure.

Many minerals are isostructural when they differ only in the nature of a cation.

Compounds which are isoelectronic usually have similar chemical structures. For example, methane, CH₄, and the ammonium ion, NH₄⁺, are isoelectric and are isostructural as both have a tetrahedral structure. The C-H and N-H bond lengths are different and crystal structures are completely different because the ammonium ion only occurs in salts.