The vanadium crossover is the primary source of capacity fade in vanadium flow batteries (VFB). Available transport models for the vanadium crossover use significantly different diffusion coefficients for the vanadium species. Previously, we demonstrated that redox reactions between V³⁺ and VO₂⁺ occur within cation exchange membranes. Therefore, it seems very likely that the neglect of redox reactions inside the membrane may lead to these inconsistencies. To investigate the reactions inside the membrane, a time-resolved in situ and in operando method based on potential probes, which have already been successfully used to measure potentials on and in electrodes, was developed. The probes were integrated into firmly pressed layers of several membranes. This work reports the new experimental design and presents the first results which give valuable insights into the transport processes and redox reactions inside the membrane.