Significant number densities of ultrafine (diameters < 200 nm) residual particles have been found from the evaporation of droplets of deionized water. Particle number densities as high as 10(6) cm(-3) have been observed, having geometric means and standard deviations of about 55 nm and 1.45, respectively. These particles are not solely due to impurities native to the water. Distributions and total aerosol loadings (in terms of both number and mass) are shown to depend on water pH, on the material of the storage container, and the storage time. Residual particles are hypothesized to result both from impurities in the water and leaching from the storage container walls. (C) 2003 Elsevier Ltd. All rights reserved.