Many distillation columns use single-end control structures in which one temperature (or composition) is controlled while the other control degree of freedom holds either the reflux ratio or the reflux-to-feed ratio constant. In most chemical processes, flow rates are determined from pressure drops over orifice plates. This means that the flow signal reflects the mass flow rate since density affects pressure drop. In computer simulations, flow rates can be studied in either molar or mass units. This paper considers the issue of using mass or molar flow ratios in single-end control structures. Results illustrate that using molar ratios can improve product quality in some systems. The practical question of how to estimate molar flow rates from plant measurements is addressed.