Isophthalic acid is an organic compound with the formula C<sub>6</sub>H<sub>4</sub>(CO<sub>2</sub>H)<sub>2</sub>. It is a white solid that is very poorly soluble in water (0.012 g/100 mL at room temperature). The main industrial uses of purified isophthalic acid (PIA) are as a comonomer in the production of polyethylene terephthalate (PET) resin and for the production of unsaturated polyester resin (UPR) and other types of coating resins.
Isophthalic acid is one of three isomeric benzenedicarboxylic acids, the others are phthalic acid and terephthalic acid. Crystalline isophthalic acid is built up from molecules connected by hydrogen bonds, forming infinite chains.
Isophthalic acid is produced by oxidizing meta-xylene using oxygen:
The approach is comparable to some routes to terephthalic acid.
The barium salt, as its hexahydrate, is very soluble in water (a distinction between phthalic and terephthalic acids). Uvitic acid, 5-methylisophthalic acid, is obtained by oxidizing mesitylene or by condensing pyroracemic acid with baryta water.