Horseradish peroxidase catalyzed polymerization of 8-hydroxyquinoline-5-sulfonate (HQS) has been studied by in-situ NMR spectroscopy. The 2, 4 and 7 positions of the HQS are involved in oxidative free radical coupling with other HQS molecules with the order of reactivity being 7 greater than or equal to 2 greater than or equal to 4. A mechanism for the oxidative free radical coupling and structure of the resulting polymer is proposed, which is supported by C-13 NMR spectroscopy.