This study provides the first in-situ characterization of the products in the heterogeneous reaction 2NO(2)(g) + H2O(a) --> HONO(g) + HNO3(a). Transmission FT-IR spectroscopy and UV-vis spectroscopy were used to probe the heterogeneous chemistry of gas-phase nitrogen dioxide on hydrated SiO2 particles. At 298 K, gaseous nitrogen dioxide reacts with adsorbed water on the surface of SiO2 particles to form adsorbed nitric acid and gas-phase nitrous acid. FT-IR spectroscopy of the hydrated SiO2 surface following reaction with NO2 provides definitive identification of the adsorbed nitric acid product. The reaction does not occur when gas-phase nitrogen dioxide is reacted on dehydrated silica particles. The gas-phase product nitrous acid was detected with both FT-IR and UV-vis spectroscopy. Although homogeneous and heterogeneous reactions can contribute to die observed HONO signal, flow cell measurements can be used to distinguish between heterogeneous and homogeneous reaction pathways leading to HONO production.