In situ digital holographic microscopy is used to characterize the dissolution flux of polycrystalline cubic tricalcium aluminate (CA-c). The surface dissolves at rates that vary considerably with time and spatial location. This implies a statistical distribution of fluxes, but an approximately steady-state median rate was obtained by using flowing solutions and by reducing the water activity in the solution. The dissolution flux from highly crystalline CA-c depends on the water activity raised to an empirically derived exponent of 5.2 and extrapolates to a median flux of - 2.1 μmol m s in pure water with an interquartile range of 3.2 μmol m s. The flux from a less crystalline source of CA-c has an empirical water activity exponent of 4.6 and an extrapolated median flux of only -1.4 μmol m s in pure water with an interquartile range of 1.9 μmol m s. These data suggest that the bulk dissolution rate of CA-c can vary by at least 30% from one source to another and that variability in the local rate within a single material is even greater because of the heterogeneous spatial distribution of structural characteristics (i.e., degree of crystallinity, chemical impurities, and defects).