In order to predict the amount of secondary organic aerosol formed by heterogeneous processing of methylglyoxal, uptake coefficients (gamma) and estimates of uptake reversibility are needed. Here, uptake coefficients are extracted from chamber studies involving ammonium sulfate and glycine seed aerosol at high relative humidity (RH >= 72%). Methylglyoxal uptake coefficients on prereacted glycine aerosol particles had a strong dependence on RH, increasing from gamma = 0.4 x 10(-3) to 5.7 X 10(-3) between 72 and 99% RH. Continuous methylglyoxal losses were also observed in the presence of aqueous ammonium sulfate at 95% RH (gamma(As,wet) = 3.7 +/- 0.8 X 10(-3)). Methylglyoxal uptake coefficients measured at >= 95% RH are larger than those reported for glyoxal on nonacidified, aqueous aerosol surfaces at 90% RH. Slight curvature in first-order uptake plots suggests that methylglyoxal uptake onto aqueous aerosol surfaces is not entirely irreversible after 20 min. Methylglyoxal uptake by cloud droplets was rapid and largely reversible, approaching equilibrium within the 1 min mixing time of the chamber. PTR-MS measurements showed that each cloud event extracted 3 to 8% of aerosol-phase methylglyoxal and returned it to the gas phase, likely by an oligomer hydrolysis mechanism.