The invention describes a composition that inhibits the polymerization of ethylenically unsaturated monomers, including at least 98% by weight of 4-tert-butylcatechol (4-TBC), 0.03% to 0.2% by weight of catechol (PC), and at least one impurity selected among 3-tert-butylcatechol, tert-butylhydroquinone, 3,5-di-tert-butylcatechol and mixture thereof, the total quantity of said impurities and of the PC being 0.1% to 0.8% by weight of said composition. The invention also describes a method for removing said polymerization-inhibiting composition present in a mixture including at least one ethylenically unsaturated monomer.