The importance of specifying the state of the inlet streams in determining the degrees of freedom (DOF) of a process is highlighted in the context of a single-stage separation unit. It is shown that, for the separation of a C-component mixture, the commonly accepted value of 2C + 6 as the DOF of an equilibrium single stage holds only if neither of the inlet streams lie on the phase envelope. The implications of this feature for the DOF of multistage processes is explored. The importance of a precise interpretation of the phase rule in arriving at the above results is emphasized.