Reflection-absorption infrared spectroscopy (RAIRS) has been employed to investigate the heterogeneous hydrolysis of bromine nitrate (BrONO2) in and on thin ice films between 95 and 185 K. Hydrolysis is observed to occur throughout the temperature range with evidence for two competing mechanisms. At the stratospherically relevant temperature of 185 K, the reaction proceeds by an ionic mechanism to give the hypobromous ion, H2OBr+, and both nitric acid di- and trihydrate. A molecular mechanism is also operative to give hypobromous acid, HOBr, and hydrated nitric acid (HNO3. (H2O)(n)). This second route becomes more important under conditions of limited water presence and at lower temperatures. All of the results are compared to analogous studies previously performed on chlorine nitrate (ClONO2).