The thermal decompositions of anhydrous fluorosilicates M2SiF6 (M=Li, Na, K, Rb, Cs) and MSiF6 (M=Ca, Sr, Ba) were investigated. The decompositions proceeded according to a simple acid-base mechanism with evolution of SiF4. The influence of the cationic counter-ion acidity (expressed by means of the electronegativity force) on the course of thermal decomposition was estimated quantitatively.