The stochastic event of Br- electrolysis in electrochemically generated ethylpyridinium polybromide (EPyBr2n + 1) on Pt ultramicroelectrode (UME) was studied. The individual current spikes of the chronoamperograms (CAs) and the estimated charges associated with electro-reductive peaks from the cyclic voltammograms (CV) revealed that Br- in EPyBr2n + 1 was electro-oxidized to Br-3(-) and further to Br-2. The experimentally estimated values regarding such electrochemistry of Br- in EPyBr2n + 1 are as follows: 1) the stability constant of Br-3(-) formation from Br- and Br-2 in EPyBr2n (+ 1) was estimated to 22.5 M-1 based on the two voltammetric curves related to BC electro-oxidation in EPyBr2n + 1; 2) the maximum concentration of Br- which can be electrolyzed in EPyBr(2)n I was estimated to ca. 15.5 ( 4.9) M by fitting the bulk electrolysis model; 3) additionally, the minimum concentration of EPyBr(aq) in acidic solution required to generate the detectable individual current spikes in CAs was estimated as ca. 32 mM. It is noteworthy that the electrochemical behavior of Br- in EPyBr2n + 1 turned out different from that in MEPBr2n + 1, which was reported previously, in that further oxidation of Br-3(-) to Br-2 was observed only in EPyBr2n (+ 1) but not in MEPBr2n + 1. This implies that the stability of Br-3(-) is strongly dependent on quaternary ammonium cations, MEP+ and EPy+. A possible explanation for such stability difference in organic media (i.e. MEPBr2n+ 1 and EPyBr2n + 1) was also provided.