The phase-separation dynamics of a ternary mixture (A, B and C) coupled with a reversible chemical reaction between the two constituents A and B is presented. It is demonstrated that the free-energy functional form of time-dependent-Ginzburg-Landau equation describing the phase-separation dynamics of the ternary mixture coupled with a reversible chemical reaction is similar to that of the mixture composed of a block copolymer and a homopolymer. Our simulation study reveals that for the case of equal forward and backward reaction rates, the lamellar thickness scales with the reaction rate constant as a single power law lambda(L)similar toGamma(-0.22), regardless of high or low reaction rate regimes. This study sheds insight to the unique features of the involvement of chemical reaction in the phase separation of the ternary mixture. If chemical reaction and phase separation take place simultaneously, the different pattern evolutions at high and low reaction rate constants are originated from the balance between the domain coarsening due to phase separation and the breakup of the continuous phase due to the chemical conversion. The different pattern evolution at high and low reaction rate constants when chemical reaction lags behind phase separation can be interpreted in terms of the discrepancy between the domain sizes at the time step immediately before the turning on of the chemical reaction and the inherent lamellar thickness. It is also pointed out that the crossover of the ternary mixture from one phase region to another, due to the concentration change between A and B. might generate interesting steady-state domain patterns.