The H2S release profiles of five important Colombian coals have been monitored using temperature programmed pyrolysis. It was found that there was no correlation between the amount of H2S and the sulphur content of the original coals. Coals which had been treated to remove all of the mineral matter and inorganic sulphur showed a good correlation with the free swelling index of the coals. This was explained by the physical trapping of H2S in closed porosity formed during pyrolysis. A similar result was found for demineralised coals with pyrite present. The H2S for untreated coals showed no systematic variation with rank, the coal sulphur content or free swelling index. This was because certain coals were rich in finely dispersed calcium which could chemically prevent H2S release.