Polycrystalline powdered ammoniojarosite was obtained by hydrolysis from bio-oxidized sulphuric pickling waste water. The kinetic parameters of the thermal decomposition process of ammoniojarosite were calculated by means of thermogravimetric measurements. Analyses were performed in non-isothermal conditions at different heating rates. The three stages of the thermal decomposition process (dehydration, dehydroxylation and deammoniation, and sulphate decomposition) are best described by the Avrami-Erofeev A(m) model. The kinetic exponent m increases, in general, with the heating rate. Apparent activation energies of 54, 244 and 130 kJ mol(-1), respectively, were obtained for each decomposition stage.