Natural and anthropogenic aerosols have a direct impact on the earth's radiative balance. Many of the common atmospheric aerosols contain hygroscopic salts and therefore their physical state and composition are sensitive to the relative humidity. The phase of the aerosol particle will dramatically affect the relative rates of atmospheric reactions, and radiative properties such as scattering and optical activity. In order to understand these atmospheric processes and provide accurate input for global modeling, it is imperative that these phase transitions be well defined. It is thought that the degree of metastability is influenced by the presence of foreign particulate matter. These solid impurities are predicted to reduce the barrier of nucleation by providing a surface upon which heterogeneous nucleation can be catalyzed. We report the results of CaCO3 and BaSO4 solid impurities on the efflorescence of (NH4)2SO4-H2O micro-droplets. The results show that both solid impurities act as catalysts for heterogeneous nucleation by causing a decrease in the degree of metastability at the efflorescence point. BaSO4 does not prove to be significantly more efficient than CaCO3 in spite of having the same crystalline structure and similar lattice parameters as in (NH4)2SO4. ¿ 1998 American Geophysical Union