Activated carbon fibres, which exhibit high specific area and numerous active surface sites constitute very powerful adsorbents and are widely used in filtration to eliminate pollutants from liquid or gaseous effluents. The fibres studied in this work are devoted to the filtration of gaseous effluents containing very small amounts (few vpm) of hydrogen sulphide. To improve their fixation capacity towards H2S the activated fibres are impregnated in an aqueous solution of potassium hydroxide and then thermally treated. The treatment leads to the deposition of crystallites of K2CO3 showing a great activity for H2S gas in the presence of water vapour. The H2S fixation mechanism proposed can be summarised as follow: K2CO3 and H2S dissolve in a liquid aqueous solution formed on the fibre surface. Then carbonate ions and H2S molecules react together almost completely to yield HS− species. This mechanism has been validated and completed by the study of the thermal effects induced when the treated fibres are in contact with H2S together with water vapour. The study has been carried out using a calorimetric method. The variations of standard enthalpy of reactions involved in the fixation mechanism are measured and compared to the data given by the thermodynamic tables for bulk solutions.