We apply high-temperature oxide melt solution calorimetry to assess the thermodynamic properties of the material Li1+xAlxTi2-x(PO4)3, which has been broadly recognized as one of the best Li-ion-conducting solid electrolytes of the NASICON family. The experimental results reveal large exothermic enthalpies of formation from binary oxides (ΔHf,ox°) and elements (ΔHf,el°) for all compositions in the range 0 ≤ x ≤ 0.5. This indicates substantial stability of Li1+xAlxTi2-x(PO4)3, driven by thermodynamics and not just kinetics, during long-term battery operation. The stability increases with increasing Al3+ content. Furthermore, the dependence of the formation enthalpy on the Al3+ content shows a change in behavior at x = 0.3, a composition near which the Li+ conductivity reaches the highest values. The strong correlation among thermodynamic stability, ionic transport, and clustering is a general phenomenon in ionic conductors that is independent of the crystal structure as well as the type of charge carrier. Therefore, the thermodynamic results can serve as guidelines for the selection of compositions with potentially the highest Li+ conductivity among different NASICON-type series with variable dopant contents.