A series of alumina samples differing in structure (gamma, delta and alpha) and in Na content, were studied by infrared spectroscopy. The same set of samples was also studied using 2-propanol dehydration as a probe reaction and using temperature-programmed reaction of 2-propanol [S. Srinivasan, C.R. Narayanan, A. Biaglow, R. Gorte and A.K. Datye, Appl. Catal. A, 000 (1995) 000]. Transitional aluminas possess well defined hydroxyl bands whose nature and intensity is affected by alumina structure and impurity content. The most reactive aluminas exhibit a prominent hydroxyl band at 3770 cm(-1) Another characteristic of the most reactive aluminas is the formation of a band at 1622 cm(-1) after pyridine adsorption and evacuation at room temperature. Addition of Na as well as thermal treatments that transform gamma- to delta-alumina have a similar effect on aluminas, namely, an attenuation of the prominent 3770 cm(-1) hydroxyl band and the 1622 cm(-1) pyridine band. In both cases, the effect appears to be steric in nature, indicative of hindered access of the probe molecule to the reactive sites. These spectroscopy results correlate well with the trends observed in catalytic reactivity.