Crossover in Vanadium Redox Flow Batteries (VRFB) has an important impact on performance and is dependent on several physical properties, including the transference number. In this work, model-guided design of experiment was used to determine vanadium transference number in fully-hydrated ion-selective membranes, while minimizing uncertainties related to unknown or unmeasured properties. The transference number of VO2+ in Nafion 117 for varying ratios of H2SO4 to VOSO4 was measured, and the analysis showed that the transference number estimate can be obtained with 5% or less uncertainty. The VO2+ transference number sharply decreased as acid was added to the electrolyte. The variation in the transference number with the ratio of H2SO4 to VOSO4 was fit to a model to obtain the product of the ratio of vanadium-to-proton partition and diffusion coefficients. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.