4-Methylphthalic anhydride is a solid organic compound that represents an isomeric form of methylphthalic anhydride and is primarily used as a reactive chemical intermediate in synthetic and industrial chemistry, where its phthalic anhydride core bearing a methyl substituent confers defined reactivity, and it is commonly encountered as a white to yellowish crystalline powder that requires careful handling and controlled laboratory procedures due to its documented potential to cause skin and eye irritation during manufacturing, formulation, and downstream chemical processing workflows.