Hybrid Ultramicroporous Materials (HUMs) are porous coordination materials with exemplary gas sorption and separation characteristics, but relatively poor thermal stability when compared to other porous coordination polymers or metal-organic frameworks (MOFs). The origin of this poor thermal stability has not yet been experimentally verified. Therefore, we investigate the thermal decomposition mechanisms of representative HUMs with the general formulae [M(SiF6)(L)2] or [M(SiF6)(L)(H2O)2], where M = Ni(ii), Cu(ii) or Zn(ii) and L = pyrazine or 4,4'-bipyridine. We find that two decomposition mechanisms dominate: (i) the fragmentation of the XF62- pillar into gaseous XF4 and fluoride, and (ii) direct sublimation of the N-donor ligand. The former process dictates the overall thermal stability of the material. We also demonstrate that HF is a possible decomposition product from certain hydrated HUM materials.