Mn1-x-yAlxCyforms a ferromagnetic metastable tetragonal τ-phase in the composition range0.42 leq x leq 0.47and0 leq y leq 0.021. Its high unaxial magnetocrystalline anisotropy makes it an interesting candidate for a permanent magnet. In order to investigate the kinetics of the transformations which occur in this system, we established the time-temperature-transformation diagram of a ternary alloy with the composition Mn0.538Al0.446C0.016. It was homogenized for 2 h at 1375 K, quenched, and subsequently isothermally annealed at temperatures between 625 K and 1025 K for various times ranging from one minute up to seven days. The quantitative phase analyses were performed by means of x-ray diffraction and stereological analysis of the microstructures. The addition of carbon increases the stability of the τ-phase, thereby allowing longer annealing times for the formation of single-phase samples.