The catalytic properties of niobic acid (Nb2O5 center dot nH(2)O) and niobium phosphate (NbOPO4) surfaces were studied in the reaction of fructose dehydration carried out in water. The reaction was performed in a continuous reactor at different temperatures 90-110 degrees C and pressures (from 2 to 6 bar). Superior activity and selectivity to 5-hydroxymethyl-2-furaldehyde (HMF) of niobium phosphate compared to niobic acid was observed. The initial higher catalytic performance of niobium phosphate than niobic acid could be related to the higher effective acidity of its surface, as was evidenced by acid-base titrations realized in different polar liquids. Deactivation studies showed a deeper decay for niobium phosphate than niobic acid at short time on stream, while, for higher time, light losses of activity were observed for both the catalysts. (c) 2006 Elsevier B.V. All rights reserved.