Lytic polysaccharide monooxygenases (LPMOs) are industrially important copper-dependent enzymes that oxidatively cleave polysaccharides. We studied two closely related AA9-family LPMOs from Lentinus similis (LsAA9A) and Collariella virescens (CvAA9A). LsAA9A and CvAA9A cleave a range of polysaccharides, including cellulose, xyloglucan, mixed-linkage glucan, and glucomannan. LsAA9A additionally cleaves isolated xylan substrates, the first LPMO to show such activity. Insights into the determinants of specificity come from the structures of CvAA9A and of LsAA9A bound to cellulosic and non-cellulosic oligosaccharides. EPR spectra further reveal differences in copper co-ordination on binding of xylan compared to glucans. LsAA9A activity is notably less sensitive to reducing agent potential on xylan when compared to other substrates, suggesting a different mechanistic pathway for the cleavage of xylan. These data show that AA9 LPMOs can display different apparent substrate specificities dependent upon both productive protein:carbohydrate interactions across a binding surface and also electronic considerations at the copper active site.