The storage reactions of the zinc/bromine battery are the cathodic deposition of zinc and the anodic formation of a nonaqueous polybromide phase. Quaternary ammonium cations, N-methylethylpyrrolidinium (MEP+) and N-methylethylmorpholinium (MEM+), store the bromine as polybromide complexes. Th mechanism of this complicated reaction determines the polarization and self-discharge rate of the bromine electrode. Electrochemical in sit techniques, phase-stabilized electrochemical quartz microbalance. and in situ reflection-absorption Fourier transform infrared spectroscopy were employed for the first time to investigate these electrode processes. It was shown that specifically adsorbed polybromide anions (Br-n(-)) formed MEM-Br-n. and MEP-Br-n ion pairs at gold electrodes, and that MEM-Br-n was adsorbed more strongly than MEP-Br-n. Therefore, a homogeneous chemical reaction of the dissolved MEP+ cation with electrochemically generated bromine leads to the storage complex MEP-Br, much more rapidly than the heterogeneous electrochemical reaction of the strongly adsorbed MEM+ to MEM-Br-n. These results demonstrate that in situ techniques not only support the evaluation of the mechanism but also provide key information for battery development.