In this paper, we report observations on single internally mixed ammonium sulfate/calcium carbonate microparticles. Deliquescence and efflorescence relative humidities for this mixed system are reported. The slightly soluble.CaCO3 has a negligible effect on the deliquescence relative humidity and the concentration dependent water activities. Mixed particles at 298 K were observed to exist in two metastable Liquid states: (a) supersaturated with respect to both ammonium sulfate and calcium carbonate and (b) supersaturated with respect to ammonium sulfate and saturated with respect to a solid calcium carbonate inclusion. The efflorescence relative humidities differed greatly between the two cases. Solution droplets supersaturated with respect to both salts (no solid inclusion present) effloresced at the homogeneous efflorescence point of pure ammonium sulfate. In contrast, the presence of the calcium carbonate as a similar to 500 nm solid inclusion within the 15 mum solution droplets served as a catalyst for heterogeneous nucleation and the particle crystallized at similar to 50% relative humidity (RH), a full 13% RH higher than homogeneous efflorescence. A smaller calcium carbonate seed induced efflorescence at an intermediate 47% RH, indicating that the heterogeneous efflorescence rate is a function of the size of the inclusion The data are analyzed using classical nucleation theory to derive the free-energy barrier to nucleation, the size of the critical nucleus, and the contact parameter between ammonium sulfate and calcium carbonate. The results are compared with available literature. The heterogeneous efflorescence was also investigated as a function of temperature (210-298 K) and found to have only a slight temperature dependence, similar to previous homogeneous efflorescence observations.