The effect of aqueous bromine/sodium bromide solutions on the stability of polystyrene and four different sulphonic acid based membranes has been investigated. It is shown that in the presence of these environments, the sulphonate groups that are directly bonded to the benzene rings are susceptible to rapid replacement by bromine. thus reducing the membrane performance. while the rate of degradation is critically dependent on the activity of free bromine. This is dependent on the ratio of bromine to bromide and on temperature. The bulk of the investigation was completed using ATR-FTIR spectroscopy, however as the area of analysis is limited to the surface, of the sampless, fluorescence microscopy and spectroscopy was Utilised to study a limited number of samples. These techniques clearly showed that degradation occurred through the thickness of the materials and is not limited to their surfaces. This result was shown to be the case for all membranes where die sulphonate group was directly bonded to the phenyl ring and it is proposed that degradation occurs via a substitution reaction mechanism. where the bromine present in solution acts as a Lewis acid. On the other hand. membranes where the sulphonate group is directly attached to a fluorinated linear chain, i.e. perfluorosulphonic acid type membranes, are stable in the bromine environment. (C) 2004 Elsevier B.V. All rights reserved.