Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that perform oxidative cleavage of recalcitrant polysaccharides. We have purified and characterized a recombinant family AA9 LPMO from Gloeophyllum trabeum, GtLPMO9B, which is active on both cellulose and xyloglucan. Activity of the enzyme was tested in the presence of three different reductants: ascorbic acid, gallic acid and 2,3-dihydroxybenzoic acid (2,3-DHBA). When using standard aerobic conditions typically used in LPMO experiments, the former two reductants could drive LPMO catalysis whereas 2,3-DHBA could not. In agreement with the recent discovery that H2O2 can drive LPMO catalysis, we show that gradual addition of H2O2 allowed LPMO activity at very low, sub-stoichiometric (relative to products formed) reductant concentrations. Most importantly, we found that while 2,3-DHBA is not capable of driving the LPMO reaction under standard aerobic conditions, it can do so in the presence of externally added H2O2.At alkaline pH, 2,3-DHBA is able to drive the LPMO reaction without externally added H2O2 and this ability overlaps entirely with endogenous generation of H2O2 by GtLPMO9B-catalyzed oxidation of 2,3-DHBA. These findings support the notion that H2O2 is a co-substrate of LPMOs, and provide insight into how LPMO reactions depend on, and may be controlled by, the choice of pH and reductant.