In order to better assess the role of nitrous acid in the chemistry of the upper troposphere and in the plumes of jet aircraft, we have studied the interaction of HONO with sulfuric acid liquid solutions (50-95 wt %) and with ice. The experiments were carried out using a Knudsen cell over the temperature range of 180-200 K for ice and 200-273 K for the solutions of H2SO4. The rate of uptake of HONO on sulfuric acid is found to vary greatly as a function of the weight percent of the solution, with an uptake probability that changes from about 0.1 at 95 wt % to less than 10(-3) for solutions under 60 wt %. HONO is adsorbed reversibly onto ice with submonolayer coverage. In the presence of HCl, the formation of ClNO is observed for both heterogeneous systems. The maximum uptake probability of HCl in the presence of HONO on the H2SO4 liquid solutions is observed at 60 wt %. The reaction takes place on ice with greater efficiency compared to H2SO4 solution experiments, which allows for the unambiguous detection of ClNO as the reaction product. We will show the results of experiments conducted over a wide range of initial conditions (reactant concentration, temperature, water partial pressure, substrate preparation) using both steady-state and realtime detection schemes. From the data, we can draw some conclusions concerning the interfacial nature of the reaction mechanism. A few brief comments on the atmospheric implications of our results conclude the discussion.