A process for purifying a nitrile such as acrylonitrile is disclosed. The process comprises (1) contacting a product mixture, which comprises acrylonitrile and hydrogen cyanide, with an acid or a precursor of the acid to produce an acid-treated product; and (2) recovering the nitrile from the acid-treated product. The product mixture can be produced by contacting ammonia with a hydrocarbon such as an olefin under a condition effective to produce a nitrile and hydrogen cyanide.