Models based on concentrated solution theory are developed to estimate from anion-tracer and cation-tracer experiments. Markov Chain Monte Carlo (MCMC) algorithms are used to estimate the probability distribution functions for accounting for uncertainties in other physical properties and experimental measurement. Under well-designed experimental conditions, can be estimated precisely, and its value is consistent with estimates based on dilute-solution theory. The assumptions made in the concentrated solution theory model are tested with measurements of methanol flux, as a tracer for water flux. It is estimated that = 0.935±0.005 for LiCl in Nafion 117 from anion-tracer experiments, while the estimates from cation-tracer experiments give = 0.938±0.035.