A humidograph with an air flow rate of 0.4 m s\\ was built to investigate the uptake of water and its effect on sizing, collection and light scattering of ambient aerosol. In the humidograph the relative humidity (RH) can be scanned over a large RH trajectory. Its performance was assessed with laboratory particles of ammonium nitrate, ammonium sulfate and sodium chloride that are the major hygroscopic components of ambient aerosol. The increase in size at the deliquescence points, which ideally is a stepwise function of RH, occurs over a range of 3% RH units. This is shown to be an optimum value in a system of such large dimensions. Because the vapor pressure of ammonium nitrate increases signi"cantly with temperature, its evaporative loss was investigated as a function of heating/drying temperature. The loss of pure test aerosol, with a mass distribution similar to that in the ambient atmosphere, was found to be acceptable for drying temperatures of up to 403C. The sizing of deliquesced aerosol by LAS-X monitors was tested and found to be a complex function of RH. In Berner low-pressure impactors growth of hygroscopic aerosol was not observed, not even at an RH approaching saturation.