The thermal decomposition of anhydrous fluoroborates MBF4 (M=Li, Na, K, Rb, Cs) and M(BF4)2 (M=Ca, Sr, Ba) was investigated. The decomposition proceeds according to a simple acid-base mechanism with evolution of BF3. The influence of the acidity of cationic counterions, expressed by the electronegativity force on the course of thermal decomposition was estimated quantitatively.