The growing applications of carbon nanotubes (CNTs) inevitably increase the risk of exposure to this potentially toxic nanomaterial. In an attempt to address this issue, research has been implemented to study the biodegradation of CNTs. In particular, myeloperoxidase (MPO), an enzyme expressed by inflammatory cells of animals including humans, catalyse the degradation of oxidized carbon nanomaterials. While reactive intermediates generated by MPO efficiently degrade oxidized single-walled carbon nanotubes (o-SWCNTs); the exact mechanism of enzyme-catalysed biodegradation remains ambiguous. In this work, we tried to explain enzymatic oxidation in terms of redox potentials by employing competitive substrates for MPO such as chloride, which is oxidized by MPO to form a strong oxidant (hypochlorite), and antioxidants that have lower redox potentials than CNTs. Employing transmission electron microscopy, Raman spectroscopy, and vis-NIR absorption spectroscopy, we demonstrate that the addition of antioxidants, L-ascorbic acid and L-glutathione, with or without chloride significantly mitigates MPO-catalysed biodegradation of o-SWCNTs. This study focuses on a fundamental understanding of the mechanisms of enzymatic biodegradation of CNTs and the impact of antioxidants on these pathways.