This study focuses on the heterogeneous reactions of gas phase glyoxal with aerosols of glycine, the most abundant amino acid in atmospheric aerosols, as well as with a mixture of glycine and ammonium sulfate (AS) at a molar ratio of 1:100 (glycine-AS 1:100). Aerosols were exposed to varying relative , humidity (RH) conditions in the presence of gas phase glyoxal for similar to 1 h, followed by drying and efflorescence. The changes in size, chemical composition, and optical properties were consequently measured. The reactions occur over a wide range of relative humidities, from similar to 30% up to 90% RH, covering values that are substantially lower as well as above the deliquescence point of the investigated aerosols. The product aerosols exhibit a trend of increasing growth in size, in optical extinction cross sections, and in extinction efficiencies (at lambda = 355 nm) with decreasing seed aerosol size, and with decreasing RH values from 90% to similar to 50%. For glycine-AS 1:100 particles, the ratio of the geometric cross section of the product aerosol to the original seed aerosol reached a value of similar to 3 the optical extinction cross section ratio was up to similar to 25, and the Q(ext) ratio was up to similar to 8, exceeding those of both AS and glycine separately, suggesting a synergistic effect. Aerosol mass spectrometer analyses show that the main products of all the studied reactions are glyoxal oligomers (light scattering compounds), with a minor contribution from imidazoles (absorbing compounds at lambda = 355 nm). These findings imply that the changes in the optical properties are likely due to enhanced scattering by the reaction products. The fraction of absorbing substances in the reacted aerosol increases with increasing RH, suggesting that the absorption component may become more substantial after longer reaction times, possibly in cloud or fog droplets. The results suggest that these reactions are possibly important in low RH regions, plausibly due to the reaction occurring in a few interfacial monolayers of water well before deliquescence.