Various samples of alumina were compared to study the effect of crystallinity and sodium impurity on alumina morphology and basicity. Carbon dioxide adsorption, followed by FT-IR and desorption (TPD), and COS hydrolysis test reaction were used. The decrease of surface area observed on rather amorphous samples after introduction of small amounts of sodium is ascribed to a better crystallinity due to migration of a part of Na+ into the bulk. The charge compensation phenomenon decreases the number of defects in the anion subnetwork. It also provokes a decrease of the number of OH groups which could explain the decrease of activity towards COS hydrolysis of the samples containing very small amounts of Na2O (<1000 ppm). For larger amounts, COS conversion and carbon dioxide experiments showed an increase in basicity.