Calcium is an essential mineral for the organism. Numerous calcium sources are available to enrich food but only calcium carbonate has been well studied. Calcium bioaccessibility depends mainly on solubility. Furthermore in vitro digestion allows to determine bioaccessibility and to identify the key parameters affecting it. This study focused on the influence of the gastro-intestinal pH, the food structure and the calcium source on the bioaccessibility of the nutrient. Four calcium sources (calcium carbonate, calcium citrate malate, dairy calcium phosphate and calcium bisglycinate) were added at 0.5% (w/w) in water (control), and in whey protein matrices as the liquid form or with heating to induce a gel. These matrices were digested under static conditions in duplicate, with the harmonized INFOGEST protocol [1]. Soluble and ionic calcium, and pH kinetics were followed during in vitro digestion. The kinetics were analyzed by nonparametric tests with repeated measurements. Results showed that all calcium sources were soluble in the matrices, mainly as ionic form (Ca2+), during the gastric phase at pH 3, but that calcium become insoluble in the intestinal compartment at pH 7. This rise in calcium insolubility led to a fall of the nutrient bioaccessibility from 20 to 80%. Dairy calcium phosphate had a distinct behavior with an important fall of solubility in the intestinal compartment due to strong ionic interactions between calcium and phosphate. In addition, the organization of whey proteins in the food matrix had no effect on solubilisation of calcium and therefore on its bioaccessibility. These results provide new insights to better understand bioavailability differences between calcium sources and matrices. Further in vivo digestion experiments on a pig model will be achieved in order to confirm bioaccessibility and to measure the bioavailability of those different sources of calcium. It will finally provide a relation between the results from in vitro and in vivo models of digestion.