The uptake of gas-phase ammonia by sulfuric acid surfaces was measured as a function of temperature (248-288 K), gas-liquid interaction time (2-15 ms), and acid concentration (20-70 wt % H2SO4) using a droplet train apparatus. The uptake coefficient increases as a function of acid concentration and reaches unity at about 55 wt % H2SO4. The increased NH3 uptake in acid solution is apparently due to reaction between NH3 and H+ at the gas-liquid interface. The results yielded parameters required to model the reaction of NH3 with H+ at the gas-liquid interface. These uptake experiments were expanded to include a detailed study of gas transport to a moving train of droplets. An analysis of previous sulfuric acid aerosol. neutralization experiments shows that the uptake of ammonia by ternary NH3-H2SO4-H2O solutions is significantly lower than that by fresh binary H2SO4-H2O solutions. At typical tropospheric water and ammonia vapor concentrations, NH3 uptake coefficients need to be included in detailed microphysical models of sulfuric acid aerosols.