A robust and highly efficient procedure to solve the isothermal two-phase flash problem at critical and sub-critical conditions is proposed in this work. From a calculated initialization, an accelerated successive substitution method is applied until either the solution is achieved or the change in the Gibbs free energy becomes positive, i.e. the Gibbs energy increases. From this point, an unconstrained optimization method is applied where the Hessian is kept positive definite and a line search method is implemented to guarantee a decrease in the Gibbs function. Using cubic equations of state to calculate the required thermodynamic properties has broadly tested the approach. Three hydrocarbon binary systems and a five-component mixture are used in this work to show the robustness of this procedure.