The uptake of gas-phase formaldehyde, CH2O, by aqueous sulfuric and nitric acid droplets has been measured as a function of temperature, acid concentration, and gas-droplet contact time. The results are interpreted in terms of the mass accommodation coefficient for formaldehyde on aqueous surfaces, the Henry’s law constant for formaldehyde in aqueous acid solutions, and the liquid-phase formation kinetics of methylenediol, CH2(OH)(2), and protonated formaldehyde, CH3O+. Time-dependent uptake studies under mildly acid solutions where CH2(OH)(2) and CH3O+ formation is slow reveal the existence of a chemisorbed surface complex for CH2O at the gas-liquid interface. Uptake studies on nitric acid and mixed sulfuric acid/nitric acid solutions show slightly enhanced uptake relative to sulfuric acid only solutions. This observation has been attributed to variation of formaldehyde solubility (expressed as Setchenow coefficients) and CH3O+ equilibrium constant in nitric and sulfuric acid solutions. The implications of these measurements for the aqueous acid chemistry of formaldehyde and its role in atmospheric chemistry are discussed.