The hydroxyl groups associated with the Brönsted acid sites in alumina are more influential in adsorbing sulfur dioxide than the more common Lewis acid sites. Hence alumina in various slightly hydrated, acid-soluble forms, such as boehmite or gamma-alumina, are far more efficient than the high-temperature corundum or a-alumina. Catalyst pretreatment with HF diminished its activity because the incorporation of halogens in the alumina lattice enhanced its Lewis acidity and blocked the OH- groups in the Brönsted sites. In the temperature range 450-600 degrees C, the catalytic activity of alumina was practically unaffected by pretreatment with an organic base, such as quinoline, which normally blocks the Lewis acid sites. Elemental sulfur activates the alumina surface during the catalyst break-in period.