Mesenchymal stem cells in combination with biphasic calcium phosphate ceramics, hydryoxyapatite/tricalcium phosphate ceramics, have been shown to induce bone formation in large, long bone defects. Gaining an understanding of the factors that may influence stem cell differentiation on these ceramics is important for developing improved scaffold materials. The objective of this study was to characterize the dissolution/precipitation behavior of calcium from this ceramic in conditions that simulate MSC differentiation into bone cells in vitro. 20/80 and 60/40 HA/TCP ceramics were immersed in control media (DMEM, 10% fetal bovine serum), OS media (control media containing dexamethasone, β-glycerophosphate, and ascorbic acid), or OS without dexamethasone. Ceramics were maintained in the media for up to 28 days at 37°C, 5% CO2, 95% humidity. The calcium level in the media was determined at days 4, 7, 14, 21 and 28 days. Calcium levels in the control media containing either ceramic dropped significantly over time as compared with control media prior to the immersion of the ceramic (day 0). The calcium level in the media containing 20/80 HA/TCP decreased the least as compared to 60/40 ceramics. The loss in calcium from the media indicates precipitation on the ceramic surface. These reactions may have an effect on MSC growth and differentiation.