The heterogeneous reactions of N2O5 with HCl and HBr on ice have been studied in the temperature range 180 to 200 K using a Knudsen flow reactor.The uptake of N2O5 on ice in the presence of HBr was found to be strongly dependent on the HBr concentration. For the highest HBr concentrations used a maximum uptake coefficient of N2O5 of γ=0.11 has been determined. We observed Br2 and HONO in 80% yield as products with respect to N2O5 taken up. The uptake coefficient of N2O5 on ice in the presence of HCl was found to be 3.2·10−2 and increased with increasing HCl concentration. CINO2 was detected as the sole reaction product with a maximum yield of 63% with respect to N2O5 consumed.Hydrolysis of N2O5 resulting in HNO3 was found to be competitive with the title reaction. For the case of the HBr reaction the branching ratio between Br2 and HONO formation, on the one hand, and hydrolysis of N2O5, on the other hand, has been determined. Mechanistic aspects of the heterogeneous reaction of N2O5 with HX have been discussed.