Recent studies have shown that niobium oxide promotes the conversion of glucose and xylose, respectively, to 5-hydroxymetylfufural (HMF) and furfural with moderate or poor selectivities. Herein, we show that niobium oxide acts differently in both reactions. For the conversion of glucose, Nb2O5 is an efficient Lewis acid catalyst and an inefficient Bronsted acid. Interestingly, it is more active in the epimerization to mannose than in the isomerization to fructose. It is shown that associating Nb2O5 with a Bronsted acid catalyst (such as HCl or Amberlyst (R)), the selectivity to HMF can reach up to 50%. Improving further this selectivity is challenging since Nb2O5 has high selectivity to mannose, which is converted to HMF with low selectivity (comparable to glucose). For the xylose conversion, niobium oxide behaves as an actual bifunctional Lewis/Bronted acid catalyst, being efficient in the isomerization/epimerization and in the dehydration. Hence, no additional Bronsted acid catalyst is required. Furfural selectivities of up to 46% are obtained, which is comparable to catalytic systems previously reported using mineral acids. Catalyst stability studies have shown that Nb2O5 has similar deactivation in water and organic solvent, furthermore, the stability can be improved further if niobium oxide is dispersed on a silica support.