Gas turbine engine operating temperatures are currently limited by the penetration of molten sandy deposits, scientifically represented in the CMAS (Ca-, Mg-, Al-, and Si-oxides) system, into the thermal barrier coating (TBC). Reactive crystallization, involving dissolution of rare-earth oxide from the TBC into the CMAS melt, is the favored mitigation strategy for this penetration. Dissolution leads to the formation of different crystalline phases, reducing the amount of melt and ultimately the penetration depth into the columnar TBC structure. Recent studies have shown that apatite is a promising phase for reactive crystallization, due to its rapid nucleation and high amount of melt constituents. However, the influence of different rare-earth oxides on the crystallization kinetics of apatite has not been investigated; in this work, rare-earth oxide/CMAS mixtures were probed using differential scanning calorimetry (DSC) to yield activation energies and pre-exponential factors of different apatite compositions. The activation energy and the pre-exponential factor will allow us to establish a qualitive comparison of which rare-earth oxide is most likely to form apatite when dissolving into a CMAS melt. Further research building on this work can lead to the best possible selection of rare-earth oxide for TBCs to protect against melt penetration.