Flow batteries with vanadium and iron redox couples as the electroactive species are employed to investigate the transport behavior of vanadium ions in the presence of an electric field. It is shown that the electric field accelerated the positive-to-negative and reduced the negative-to-positive transport of vanadium ions in the charging process and affected the vanadium ion transport in the opposite way during discharge. In addition, a method is designed to differentiate the concentration-gradient-driven vanadium ion diffusion and electric-field-driven vanadium ion migration. A simplified mathematical model is established to simulate the vanadium ion transport in real charge–discharge operation of the flow battery. The concentration gradient diffusion coefficients and electric-migration coefficients of V2+, V3+, VO2+, and VO2 + across a NAFION® membrane are obtained by fitting the experimental data.