The COS hydrolysis reaction (COS + H2O = CO2 + H2S) using gamma-Al(2)O(3)at 220 degrees C is described and discussed. The stoichiometric reaction of COS in the absence of water proceeds and the extent of reaction is comparable to the moisture content of the alumina that can be removed during TGA. The reaction of COS and H2O is long lived at this temperature but the sulphur balance based on H2S is ca. 80%. It is found that elemental sulphur is deposited in the reactor and on the catalyst during this reaction. Reaction in the presence of H-2 increases the sulphur balance to 100% and no sulphur is deposited on the catalyst. The beneficial effect of hydrogen is demonstrated by mixing sulphur with gamma-Al2O3 which on reaction with H-2 leads to the formation of COS and H2S. The addition of formic acid to the reactants leads to a reversible loss of activity, but detailed long term studies show that formic acid is not formed during the operation of the catalyst over a wide range of conditions.