Three different transitional aluminas were doped with controlled and measured amounts of Na and were tested for 2-propanol dehydration activity. These alumina samples were activated by heating at 773 K in situ before use. In order to examine the sites responsible for the reaction, simultaneous temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) measurements of 2-propanol were performed on the samples. A typical gamma-alumina sample adsorbed ca. 2 . 10(18) molecules of 2-propanol per m(2). When 400 ppm (0.04 wt.-%) of Na was added to this sample, the reactivity fell by 75%. Since the Na level corresponds to 0.07 . 10(18) atoms/m(2), this suggests that the active sites are much fewer in number than the number of adsorption sites. Besides affecting the active sites, Na also modifies the adsorption sites causing a fraction of the adsorbed 2-propanol to desorb before reaction in the TPD experiment. The effect of added Na was contrasted with the effect of alumina form (gamma, delta and alpha). These alumina samples were also characterized by IR spectroscopy in a companion study (part II of this paper, Appl. Catal. A, 000 (1995) 000) where we found that the added Na impurities affected the hydroxyl groups and adsorbed pyridine. This study provides a comprehensive picture of how Na impurities modify the catalytic behavior of transitional alumina.