Various methods developed to measure the lithium ion transference number in solid polymer electrolytes have given widely varying results. This may be explained by invalid assumptions made in the analyses of the transport experiments, such as the assumption of an ideal, dilute solution. One popular method, referred to as the steady-state current method, and based on the potentiostatic polarization of a symmetric cell, is analyzed here for a binary electrolyte under concentrated solution theory. The ratio of steady state to initial currents after the imposition of a potential step is shown to involve all the transport properties as well as the mean molar activity coefficient of the salt. This method is compared to other methods that utilize the current/voltage response of a symmetric cell, and underlying similarities in the methods are revealed.