The recently discovered lytic polysaccharide monooxygenases (LPMOs) are Cu-containing enzymes capable of degrading polysaccharide substrates oxidatively. Here we have monitored the Cu-site geometry in two AA9 LPMOs, one from Lentinus similis (LsAA9_A) and one from Thermoascus auranti­acus (TaAA9_A), as the active-site Cu is photoreduced in the X-ray beam. In addition, we have characterized in detail the structural effects of oligosaccharide binding in differently photoreduced states of LsAA9_A.