The SO2 retention capacity of different activated carbon fibres (obtained from pitch, PAN and phenolic resin precursors) was analysed. The experiments were carried out in a fixed bed reactor at a temperature of 100 degrees C. A model flue gas with a composition of 3000 ppm SO2, 5% O-2, 10% H2O and N-2, was used in all experiments. The effect of different chemical (NH3 at 800 degrees C, HNO3 (at reflux), HNO3 (reflux)/NH3 (at 250 degrees C), O-2/NH3 (at 450 degrees C)) and thermal treatments to enhance the SO2 capture capacity was studied. The PAN-based activated carbon fibres showed the best SO2 retention characteristics. Treatment with ammonia at 800 degrees C has proved to be an efficient way to increase the retention capacity of activated fibres. The SO2 capture capacity increases with the total surface basicity of activated carbon fibres. The retention capacity markedly decreases after various retention-regeneration cycles. This reduction on sulphur retention can be partially avoided by increasing the regeneration temperature from 600 degrees C to 800 degrees C.