Calcium is an essential mineral for the organism. Numerous calcium sources are available to enrich food products, 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. The purpose of this study was to focus on the influence of gastro-intestinal pH variation, the effect of the food matrix structure and the calcium sources on calcium bioaccessibility. 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 liquid or heat-induced gel. These matrices were digested under static conditions in duplicate, with the harmonized INFOGEST protocol (Minekus et al. 2014). 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 insolubilisation occurred in the intestinal compartment at pH 7. Dairy calcium phosphate insolubilisation was quicker and more intense than for other calcium sources; its concentration of soluble calcium is 17+/-3%, instead of more than 60% for the three other calcium sources (p<0.001). In addition, gelation of whey proteins had not impact on calcium bioaccessibility (liquid vs. heat-induced gel: p>0.1). 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.