Laboratory studies are described that suggest reactive uptake of glyoxal on particulate containing HNO3 could contribute to the formation of secondary organic aerosol (SOA) in the upper troposphere (UT). Using a Knudsen cell flow reactor, glyoxal is observed to react on supercooled H2O/HNO3 surfaces to form condensed-phase glyoxylic acid. This product was verified by derivatization and GC−MS analysis. The reactive uptake coefficient, γ, of glyoxal varies only slightly with the pressure of nitric acid, from γ = 0.5 to 3.0 × 10−3 for nitric acid pressures between 10−8 and 10−6 Torr. The data do not show any dependence on temperature (181−201 K) or pressure of glyoxal (10−7 to 10−5 Torr). Using the determined reactive uptake kinetics in a simple model shows that glyoxal uptake to supercooled H2O/HNO3 may account for 4−53% of the total organic mass fraction of aerosol in the UT.