Solid acid NbOx/ZrO2 catalysts containing 0, 0.5, 1.1 and 2.8 wt.% of Nb were prepared for one-pot hydrolysis-dehydration of mechanically activated cellulose in pure hot water to produce glucose and 5-hydroxymethylfurfural. The catalysts were characterized (XRD, N-2 adsorption, ICP-OES, UV-Vis DRS) and tested in cellulose processing at 453 K. The catalytic activity increased as follows: 0.5%Nb/ZrO2 < ZrO2 < < 1.1%Nb/ZrO2 similar to 2.8%Nb/ZrO2. This series was accounted for by the acid-basic properties of ZrO2, by the inhibition of its active sites by monomer niobium species and by the appearance of Bronsted acidity upon the formation of NbOx-containing polymer structures. The highest glucose and 5-HMF yields (22 and 16 mol%, respectively) were achieved over 2.8%Nb/ZrO2.