The thermal decomposition of tetraalkyl ammonium tetrafluoroborates (R4NBF4), where R=methyl, ethyl, n-propyl and n-butyl, were studied using thermogravimetric (TG) technique in an inert atmosphere of N-2 at a heating rate of 20 K min(-1). The decreasing order of thermal stability and activation energy (E) with increase in size of the substituted alkyl group are consistent with each other. The probable causes for higher activation energy observed in the case of n-butyl substituted compound are discussed.