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The stability of β-Mo₂C surfaces has been computed at the level of density functional theory under the consideration of the temperature, pressure, and molar ratios of CH₄/H₂ and CO/CO₂ gas mixtures by using ab initio atomistic thermodynamic calculations. It was found that the (001) surface is most stable at low temperature, whereas (101) becomes dominant at high temperature with CH₄ as carbon source, and the computed surface stability is supported by the experimental X-ray diffraction pattern and intensity. For CO as carbon source, the (101) surface has the smallest surface Gibbs free energy at temperatures up to 1000 K and is most stable. On the basis of the Wulff-type particle shapes from surface Gibbs free energies the (101) facet represents the largest surface area of β-Mo₂C. Our findings are in perfect agreement with the results of high-resolution transmission electron microscopy.