The heterogeneous reaction of HNO3 adsorbed on porous glass surfaces with gaseous NO was investigated using transmission Fourier transform infrared (FTIR) spectroscopy at room temperature. The amount of adsorbed HNO3 varied from (4.2-110) x 10(17) molecules of HNO3 on a 6 cm(2) porous glass plate whose BET surface area was measured to be 28.5 +/- 0.6m(2) (2 sigma). The initial concentration of gaseous NO varied from (0.2-6) x 10(17) molecules cm(-3). A rapid release of NO2 into the gas phase was observed to occur simultaneously with a decrease in adsorbed HNO3. A trace amount of gaseous HONO was also formed. The measured yields of NO2 and loss of HNO3 and NO are consistent with the net reaction 2HNO(3) + NO --> 3NO(2) + H2O which is due to HNO3 + NO --> HONO + NO2, followed by HONO + HNO3 --> 2NO(2) + H2O and/or 2 x (HNO3 + NO --> HONO + NO2) followed by 2HONO --> NO + NO2 + H2O. Both (NO2)-N-15 and (NO2)-N-14 were observed as reaction products when (NO)-N-15 and (HNO3)-H-14 were used as the reagent species, indicating that some of the NO2 produced originates in HNO3. The measured decay rates for adsorbed HNO3 were first order in NO. The rates initially increase with increasing HNO3 but tend to plateau, consistent with complete surface coverage and the formation of multilayers of HNO3, perhaps in part in the pores. Extrapolation of these results to atmospheric NO levels suggests that this heterogeneous reaction may serve as a mechanism to regenerate photochemically active forms of NOx and nitrous acid from HNO3 in the atmosphere.