A thermodynamic optimization of the Mo–Re system by the Calphad method is proposed, taking into account new experimental data obtained in this work. According to recent crystal chemistry work, the χχ and σσ Frank–Kasper phases were modeled using two independent sublattices of variable occupation. This was made possible by including experimental site fractions in the different sublattices as input data for the parameter optimization. A consistent set of parameters is proposed. Good agreement is found between calculated and selected experimental values for different kinds of data: phase diagram, thermodynamic and crystallographic data.