The effect of sonication on the dehydroxylation process of antigorite has been studied using TG-DTG and FTIR spectroscopy. Sonication process causes important particle-size reductions, while the structure of the antigorite is preserved as shown by XRD and FTIR. This particle size reduction strongly influences the thermal behaviour of antigorite. The dehydroxylation of untreated antigorite occurs mainly through a stage centred at 749°C. On the contrary, the dehydroxylation of the sonicated sample occurs through three differentiated stages. Spectroscopy study shows that the first stage of dehydroxylation exclusively corresponds to the release of outer hydroxyl groups, while the other two stages at high temperatures correspond to the simultaneous evolution of both inner and outer hydroxyls.