The thermodynamics of two-phase coherent equilibrium is analysed in the case of a phase transition without change of composition. The elastic contribution to the Gibbs free energy associated with a distortion of the matrix of the parent phase due to the transformation misfit strain is considered. The phase transition from the parent to the product phase is studied using Landau theory, with the transition volume change being coupled with the phenomenological order parameter. The minimization of the free energy with respect to the volume change and order parameter gives the dependence of the Gibbs energy on the volume fraction of the product phase. The transformation proceeds in a finite-temperature region with the equilibrium volume fraction depending on temperature rather than at a fixed temperature as would be expected from the Gibbs phase rule for the first-order transition.