The thermal behaviour of double ammonium sulphate hexahydrates of Mn(II), Fe(II), Co(II), Ni(II) and Cu(II) was studied in air using TG and DTA techniques. X-ray powder diffractometry was used to identify the solid decomposition products. The thermal stability of these double salts in the solid state is discussed and the mechanism of their decomposition is also suggested. A kinetic analysis of the TG data was performed by using Jeres’ modification of the Freeman-Carroll method to determine the apparent activation energy and order of reaction.