Peracetic acid (also known as peroxyacetic acid, or PAA), is an organic compound with the formula CH3CO3H. This organic peroxide is a colorless liquid with a characteristic acrid odor reminiscent of acetic acid. It can be highly corrosive.

Peracetic acid arises upon treatment of acetic acid with hydrogen peroxide, the equilibrium constant being 0.37 at room temperature:

As an alternative, acetyl chloride and acetic anhydride can be used to generate products with lower water content.

Peracetic acid is synthetized  by continuously feeding acetic acid and hydrogen peroxide into an aqueous reaction medium containing a sulfuric acid catalyst. The reaction is allowed to continue for up to ten days in order to achieve high yields of product according to the following equation.
 

Peracetic acid is always sold in solution with acetic acid and hydrogen peroxide to maintain the stability of the chemical. The concentration of the acid as the active ingredient can vary, and usually depends on its application. Additional methods of preparation involve the oxidation of acetaldehyde or, as an alternative, as an end-product of the reaction of acetic anhydride, hydrogen peroxide, and sulfuric acid. Another method involves the reaction of tetraacetylethylenediamine  in the presence of an alkaline hydrogen peroxide solution.PAA is also formed naturally in the environment through a series of photochemical reactions involving formaldehyde and photo-oxidant radicals.

The main use of peracetic acid is for the industrial synthesis of epoxides. It transfers an oxygen atom to double bonds, e.g. in ethene and propene, to form epoxides and alcohols. It can also be used in producing synthetic glycerol from propene, and is used in nylon manufacturing