High surface area (HSA) alpha-aluminas have now been produced from both ground (G) and unground (U) diaspore. The porosities of these (HSA) alpha-aluminas were determined using nitrogen adsorption at 77 K and mercury porosimetry. Volumetric and calorimetric studies Of CO, CO2, and C2H4 adsorption at 300 K were performed to examine the surface properties of these (HSA) alpha-aluminas as well as a commercial low surface area (LSA) alpha-alumina so that comparisons could be made. A pore size distribution with a maximum at a radius of around 60 angstrom was observed with both HSA samples, but the unground sample possessed only a small quantity of mesopores (< 0.01 cm3 g-1) while the ground sample had an appreciable mesopore volume (ca. 0.07 cm3 g-1). Carbon monoxide and ethylene adsorption was completely reversible, and the corresponding heats of adsorption for carbon monoxide adsorption on the unground and ground samples were 6.1 and 15.4 kcal mol-1, respectively. The ethylene heat of adsorption on these two (HSA) alpha-aluminas was 3 kcal mol-1, which is one-third the values reported for eta-alumina. Significant amounts of irreversible carbon dioxide adsorption occurred on the (HSA) alpha-alumina samples, i.e., 106 and 101 mumol g-1 for the unground and ground samples, and the respective heats of adsorption were 21.4 and 16.0 kcal mol-1. However, on a normalized basis, the respective carbon dioxide coverages on the two (HSA) alpha-aluminas of 8.2.10(17) and 5.8.10(17) molecule m-2 were lower than the value of 9.8.10(17) for the (LSA) alpha-alumina. When compared to heat of adsorption (Q(ad)) values for transition aluminas, the low value for carbon monoxide on the unground sample indicates weaker Lewis acid sites while the higher value for the ground sample indicates acid strengths more similar to transition aluminas. The Q(ad) values for carbon dioxide adsorption are quite consistent with previous heats of adsorption on transition aluminas, and the concentration of basic sites on aluminas appears to have the following relationship : (LSA) alpha-Al2O3 > (HSA) alpha-Al2O3(U) > (HSA) alpha-Al2O3(G) greater-than-or-equal-to gamma- and eta-Al2O3.