CaC2 production is studied from the viewpoint of thermodynamics using an equilibrium composition model with total Gibbs free energy minimization. It is found that CaC2 is formed directly from the reaction of C and CaO without an intermediate step under the conditions of industrial practice. The different reaction mechanisms reported in the literature can be attributed to the difference in reaction conditions. The model determines equilibrium composition of the reaction system under various conditions, including those applicable to the autothermal CaC2 production process.