The solidification of a metastable ferromagnetic r phase in undercooled Mn-AI-C alloys is examined in terms of the relative thermodynamic phase stability and competitive kinetics. The heat of transformation AH~ ~ of the metastable phase to the equilibrium h.c.p, e phase and the heat of fusion AHf-~ of the h.c.p, e phase in a Mno.55A10433C00j 7 alloy were measured to determine the Gibbs free energy differences between the metastable and stable phases as a function of temperature. The results indicate that a minimum amount of undercooling of A T = 87 K is required to form the metastable ferromagnetic r phase over the equilibrium phases in a Mn0.55A10433C,.017 alloy. The thermodynamic evaluation was applied further to develop a complete temperature-time-transformation (TTT) diagram of the Mnll.55Alo.433Cil.l~17 alloy based on a solid-state transformation kinetics analysis of the e~ r reaction during isothermal treatments. This analysis reveals the critical cooling rate of the order of 20 K s-~ to bypass the metastable r phase nucleation during solid state transformation in agreement with empirical observations. The combined thermodynamic and competitive kinetics analysis provide a unified guide for the processing of near equiatomic Mn-AI alloys.