The development of the acidity and the relationship between acidity, catalytic activity, and the surface structure for niobium oxide supported on zirconia were investigated for a series of solids. The catalysts were active for 2-propanol dehydration only above a threshold in Nb loading. The acidity was studied by infrared spectroscopy of adsorbed 2,6-dimethylpyridine as a probe molecule, and the onset of activity was correlated with that of the formation of relatively strong Brønsted acid sites. The variation in the abundance of these sites also correlated with the catalytic activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric Nb surface species. These results were compared to those previously reported for the WO(x)/ZrO(2) catalysts.