External functions are a special feature of the modeling and optimization environment GAMS, which facilitates the integration of external routines for the solution of complex sub problems. The available external functions can be used for the solution of equilibrium calculations as well as the calculation of additional thermodynamic properties, incl. liquid and vapor enthalpies. This does not only allow for a simple integration of the different thermodynamic property models, but also results in a significant reduction of the nonlinearity and the size of the optimization problem handled in GAMS. Currently a set of external functions is available, which is suited for homogeneous and heterogeneous distillation (VLE and VLLE), LL-Extraction (LLE), as well as calculations for different thermodynamic properties for the modeling of membrane processes. The available external functions can also be used as guideline example for the development of additional external functions.