In this paper we set out the principles for using the grand composite curve (GCC) of mass exchange network synthesis (MENS) for selection of external mass separating agents (MSAs). These principles enable us to systematically choose between alternative external MSAs as well as the minimum flowrate for each MSA selected. This paper emphasizes that the cheapest MSA is not necessarily the one with the lowest cost per unit mass of MSA, but rather the one with the lowest overall cost of removal of the mass load, which depends on both the MSA cost and its permissible concentration change. It also demonstrates the importance of consideration of the composition levels of the MSAs in relation to the target compositions of the rich streams and to the capital cost implications of the resulting mass transfer driving forces. In this respect the use of the GCC is superior to procedures developed so far. We illustrate the principles presented by application to two example problems.