Potassium carbonate (K2CO3) is a promising material for the long-term storage of renewable energy. A reactor vessel filled with K2CO3 can potentially be used as a domestic heat battery. The hydration and dehydration reactions of salt hydrates in a reactor vessel are generally described using a one-process model, such as the ‘Arrhenius-f(α)’ model. However, this modeling approach cannot always be applied correctly. If the reaction does not proceed in a pseudo-steady state, and/or when nucleation and growth processes are simultaneously active during the transformation from an anhydrous to a hydrated state, the one-process modeling approach should not be applied. In this paper, it is investigated using simultaneous thermal analysis (STA) experiments whether the pseudo-steady state approximation is valid during the hydration reaction of K2CO3. Additionally, ‘jump experiments’ using STA are employed to investigate the rate-determining step (RDS) of the hydration reaction by applying step-wise changes in partial water vapor pressure. The presence of nucleation and growth processes during the hydration reaction is investigated by fitting isotropic models to STA data. The STA results showed that indeed the hydration of K2CO3 happens in a pseudo-steady state, and the reaction can be described using a RDS. An isotropic nucleation and growth model shows that the hydration reaction can be described by assuming instantaneous nucleation followed by diffusion-limited growth. This leads to the general conclusion that the one-process modeling approach, such as the Arrhenius-f(α) model, is valid to describe the hydration reaction of K2CO3 particles.