Potassium carbonate, K2CO3, has been identified as one of the most promising thermochemical storage materials for the built environment. Where a lot of knowledge has been gained on hydration/dehydration behavior at atmospheric (open system) conditions, little is known of this process under pure water vapor conditions (closed vacuum system). In this paper, for the first time, the equilibrium behavior and reaction kinetics of a K2CO3 composite are investigated under pure water vapor conditions, as present in closed vacuum systems. In this work the metastable behavior of a K2CO3 composite is investigated under vacuum conditions and compared to its metastable behavior under atmospheric conditions. It is found that the metastable zone is also present in vacuum conditions, however induction times in the metastable zone are much shorter which indicates a faster nucleation rate in vacuum conditions. Moreover the effect of inert gasses in a closed system is studied and it is shown that it is critical to remove all sources of non-condensable gasses. Finally in cyclic measurements it is shown that K2CO3 is stable in multi cyclic experiments, concluding that it is a suitable material for a heat battery based on the concept of a closed reactor.