Accurate models of atmospheric processes are currently hindered by poor microphysical models of particle phase transitions, including efflorescence. For aqueous particles, it is possible that insoluble crustal inclusions behave as heterogeneous nuclei to induce efflorescence. This paper reports on the results of laboratory experiments concerning the efflorescence of (NH4)2SO4 particles that are internally mixed with TiO2, Al2O3, or ZrO2 particles. The particles are prepared by condensing SO3 and H2O vapors in the presence of insoluble metal oxide particles, followed by neutralization with NH3. A Fourier transform infrared spectrometer is employed to observe the aqueous-to-solid phase change as relative humidity decreases. Efflorescence occurs at 35% relative humidity (¿2%) in the absence of heterogeneous nuclei. In contrast, internally mixed particles effloresce at 57% (Al2O3), 59% (ZrO2), and 65% (TiO2) at 298 K. ¿ 1999 American Geophysical Union