Mass spectroscopic results confirm that water vapour, ammonia and oxygen are the main gas products that form during the thermal decomposition of (NH4)4V2O11. From the results it is clear that the decomposition consists of a number of steps which overlap. The mass spectroscopic results fit the results as observed by thermogravimetry. From 60-degrees C, when a heating rate of 2-degrees C min-1 is used, the compound loses a molecule of water to form NH4[VO(O2)2(NH3)], which decomposes in the next step into NH4[VO3(NH3)] (from 85-degrees C). The decomposition rate of the first reaction is slower than that of the second reaction. From 125-degrees C (heating rate 2-degrees C min-1), NH4[VO3(NH3)] decomposes into ammonium meta-vanadate. This reaction seems to consist of two steps, and could include temperature-dependent mechanisms.