Abstract. Jafarey et al. (1979) derived a simple analytical expression for the separation factor S. This paper provides a simpler and more instructive derivation of this expression, and also evaluates its validity for estimating steady-state gains and RGA-values. The results show that the expression gives a good estimate of how S changes with internal flows, but describes poorly its variation with external flows. Fortunately, when computing the steady-state gains, this error is often not important.