The uptake of gas-phase acetaldehyde 3CHO, ethanal> by aqueous sulfuric acid solutions was studied under upper tropospheric/lower stratospheric (UT/LS) conditions. The solubility of acetaldehyde was found to be low, between 2 ¿ 102 M atm-1 and 1.5 ¿ 105 M atm-1 under the ranges of temperature (211--241 K) and acid composition (39--76 weight percent, wt%, H2SO4) studied. Under most conditions, acetaldehyde showed simple solubility behavior when exposed to sulfuric acid. Under moderately acidic conditions (usually 47 wt% H2SO4), evidence of reaction was observed. Enhancement of uptake at long times was occasionally detected in conjunction with reaction. The source of these behaviors and the effect of acetaldehyde speciation on solubility are discussed. Implications for the uptake of oxygenated organic compounds by tropospheric aerosols are considered.