Adulterants are chemical impurities or substances that by law do not belong in a food, pesticide, or other substance. Some are added intentionally to lower the manufacturing cost of the product, or to modify its characteristics in a deceptive way.

Usage of adulterants was very common and often was a penal offense. A few examples used through the history are:

• Mogdad coffee, whose seeds have been used as an adulterant for coffee
• Roasted chicory roots were used for the same purpose, starting during the Napoleonic era in France
• Roasted ground peas, beans, or wheat used to adulterate roasted chicory
• Diethylene glycol, used by some winemakers to fake sweet wines
• Oleomargarine or lard, added to butter
• Rapeseed oil, commonly added to sunflower oil and soybean oil, brassicasterol being a marker of its presence
• Rye flour, corn meal or potato starch used to dilute more expensive flours; alum is also added to disguise usage of lower-quality flour
• Apple jellies were substituted for more expensive fruit jellies, with added colorant and sometimes even little pieces of wood that simulated eg. strawberry seeds
• Artificial colorants, often toxic - eg. copper, zinc, or indigo-based green dyes added to absinthe
• Sudan I yellow color, added to chili powder
• Water, for diluting milk and beer
• Lower-quality black tea disguised as higher class
• Starch, added to sausages
• Cutting agents are often used to adulterate (or "cut") illicit drugs

Adulterants can be also added to urine, in order to interfere with the accuracy of drug tests. They are often oxidative in nature - hydrogen peroxide, and bleach have been used, sometimes with pH-adjusting substances like vinegar or sodium bicarbonate. These can be detected by drug testing labs, but some of the less expensive tests do not look for them.