The Br-2/Br- oxidation-reduction couple is employed as the positive electrolyte in some redox flow batteries, such as zinc-bromine, vanadium-bromine, and hydrogen-bromine batteries. However, the Br-2 and Br- crossover behavior with the proton exchange membrane is detrimental in these flow batteries. To explore the mechanism of the Br-2 and Br- crossover, a spectrophotometer and a special reagent were developed to detect the accumulative penetration of the Br-2 and Br- in the negative electrolyte. In addition, a laboratory-designed quinone-bromine flow battery was developed to investigate the effect of Br- and Br-2 crossover behavior on cell performance. What's more, molecular dynamics simulations of Br- and Br-2 passing through a membrane were performed to analyze Br- and Br-2 crossover behavior. It can be concluded that Br-2 crossover was the key factor contributing to the current inefficiency. The coulombic efficiencies increased from 72.5 to 82.5 to 90% as the current density increased from 0.025 to 0.075 A cm(-2) which occurred due to Br species crossover. The cell performance factors are strongly related to the amount of Br-2 crossover; hence, these factors will vary as the bulk aqueous phase Br species concentration changes. The Br-2 and Br- crossover mechanism with the proton exchange membrane in a quinone-bromine flow battery was detected through a spectrophotometer with a special reagent and the molecular dynamics (MD) simulations, and the related cell performance was also investigated, which is beneficial to other redox flow batteries with the employment of Br-2/Br- oxidation-reduction couple.