In order to examine the effects of ion-exchanged metals on the thermal decomposition of carboxyl groups in coal, we have quantified the distributions of oxygen-containing product gases from raw, acid-washed, and calcium-exchanged Yallourn brown coals, pyrolysed in a fluidized bed reactor at temperatures of 600 to 1050°C. The major products are CO and CO2. The minor ones are comprised of oxygenated organic species (acetic acid, ketene, formaldehyde, and methanol), whose summed yields approach that of methane from primary pyrolysis. At temperatures less than 900°C, calcium enhances the yield of CO2 and decreases the production of CO, formaldehyde, and methanol. It completely suppresses the production of acetic acid. These results are consistent with a mechanism involving decomposition of the calcium carboxylate structure to eliminate CaCO3, which takes with it some of the organically bound oxygen that would otherwise be able to form oxygen-containing organic gases. The CaCO3 decomposes in turn and is responsible for the increased yields of CO2. At higher temperatures (>900°C), decomposition of the organic species and char gasification appear to predominate, leading to high yields of CO.