Isophthalic acid (CAS No. 121‑91‑5; formula C₆H₄(CO₂H)₂; MW 166.13 g/mol) is a white crystalline powder (technical grade purity ~82–95 %) with a high melting point (~341–343 °C), minimal water solubility, and excellent thermal stability ([turn0search1], [turn0search4]). It's widely used as an industrial intermediate in resin production and polymer synthesis.