An adulterant is a chemical substance which should not be contained within other substances (e.g. food, beverages, fuels) for legal or other reasons. Adulterants may be intentionally added to more expensive substances to increase visible quantities and reduce manufacturing costs or for some other deceptive or malicious purpose. Adulterants may also be accidentally or unknowingly introduced into substances. The addition of adulterants is called adulteration.

Examples of adulteration include:
Mogdad coffee, whose seeds have been used as an adulterant for coffee
Roasted chicory roots, whose seeds have been used similarly, starting during the Napoleonic era in France (and continuing until today as a moderately popular additive for cheaper coffee)
Roasted ground peas, beans, or wheat, which have been used to adulterate roasted chicory
Diethylene glycol, used by some winemakers to fake sweet wines
Oleomargarine or lard, added to butter
Alum is added to disguise usage of lower-quality flour in expensive flours
Apple jellies, as substitutes for more expensive fruit jellies, with added colorant and sometimes even specks of wood that simulate strawberry seeds
Artificial colorants, often toxic - e.g., copper, zinc, or indigo-based green dyes added to absinthe
Sudan I yellow color, added to chili powder, as well as Sudan II, Sudan III, Sudan IV and Sudan Red G for red color
Water, for diluting milk and beer and hard drinks
Low quality black tea, marketed as higher quality tea
Starch, added to sausages
Cutting agents, often used to adulterate (or "cut" illicit drugs - for example, shoe polish in solid cannabis
Urea, melamine and other non-protein nitrogen sources, added to protein products in order to inflate crude protein content measurements
Powdered beechnut husk aromatized with cinnamic aldehyde, marketed as powdered cinnamon.
High fructose corn syrup or cane sugar, used to adulterate honey; C4 sugars serve as markers, as detected by carbon isotopic signatures
Glutinous rice coloring made of hazardous industrial dyes, as well as tinopal to make rice noodles whiter (to serve as bleach)
Noodles, meat, fish, tofu preserved with formaldehyde in tropical Asia, to prevent spoilage from the sun
Ham has been used as a thickener for peanut butter.
Water or brine injected into chicken, pork or other meats to increase their weight.