This study is focused on the evaluation of the thermal stability of a nanocrystalline Mg alloy, a relatively less explored topic. Guided by Darling et al. model which advises choosing of alloying elements based on their enthalpy of mixing and elastic enthalpy with respect to the parent element, Mo was chosen for stabilizing the nc Mg structure. High energy ball milling experiments were conducted on powders of Mg and Mo to achieve a composition of Mg–2at%Mo. Characterization of the ball milled powders using XRD and TEM indicated a lack of mixing of Mg and Mo and the microstructure was found to bear a mix of Mg and Mo phases. Kissinger's analysis using DSC yielded the activation energy of grain growth in the ball milled material as 73 kJ/mol which was similar to that of pure Mg, indicating that Mo did not alter the grain growth kinetics of Mg. The poor grain size stability of the nanocrystalline Mg–2Mo composition was also observed during spark plasma sintering studies conducted at 673, 723 and 773 K. The obtained results were evaluated in the light of the Murdoch and Schuh model and this revealed that the Mg–Mo alloy is not expected to be thermally stable due to the tendency of Mo to exist as a separate phase. This analysis appears to suggest that the Murdoch and Schuh model is more suitable for identifying thermally stable nanocrystalline compositions compared to the Darling et al. model. © 2020