The formalism of dispersive kinetics is applied for thermolysis of solids. As an example, the isothermal decomposition of basic aluminium ammonium sulphate in vacuum is analysed. The first order dispersive kinetic equation provides a highly accurate description of thermolysis processes for the three stages of decomposition : partial dehydration at temperatures below 623 K, final dehydration and removal of ammonium in the temperature range 623-873 K, and desulphurisation at temperatures above 873 K.