Lytic polysaccharide monooxygenases promote enzymatic depolymerization of lignocellulosic materials by microorganisms due to their ability to oxidatively cleave recalcitrant polysaccharides. The properties of these copper-dependent enzymes are currently of high scientific and industrial interest. We describe a previously uncharacterized fungal LPMO and show how reductants, which are needed to prime the LPMO by reducing Cu(II) to Cu(I) and to supply electrons during catalysis, affect enzyme efficiency and stability. The results support claims that H 2 O 2 is a natural cosubstrate for LPMOs by demonstrating that when certain reductants are used, catalysis can be driven only by H 2 O 2 and not by O 2 . Furthermore, we show how auto-inactivation resulting from endogenous generation of H 2 O 2 in the LPMO-reductant system may be prevented. Finally, we identified a reductant that leads to enzyme activation without any endogenous H 2 O 2 generation, allowing for improved control of LPMO reactivity and providing a valuable tool for future LPMO research.