Phthalic anhydride is the organic compound with the formula C₆H₄(CO)₂O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commercially. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2000, the world wide production volume was estimated to be about 3 million tonnes per year.