In the human body calcium is the most abundant mineral and is involved in a lot of vital processes. Calcium deficiency occurs when there is not enough calcium present in the bloodstream. In the Netherlands one in three women will face this problem in their lifetime. A major factor contributing to this problem is that the intake of calcium on average is below the recommended amount which is around 950 mg/day for adults in the Netherlands. Calcium deficiency when left unattended can lead to osteoporosis. Therefore it is important to consume enough calcium rich products or take calcium supplements. Milk is a good source of calcium and is considered a staple food. This makes bovine milk a good candidate for calcium fortification. Milk can be fortified by using calcium salts to increase the bioavailability the calcium present. In this thesis in vitro methods were used to study the effects of calcium salt fortification on the phase distribution of calcium in bovine skimmed milk under different pH conditions and by using the radioactive isotope 45Ca to track the exchange between the phase casein micelles, serum proteins and the soluble phase when fortifying the milk. For unfortified milk it was found that 36.0±0.9% was present in the soluble phase, 3.7±2.9% was present in the serum proteins phase and 60.3±3.0% in the casein micelles. Acidifying the milk to simulate the gastric environment resulted in an increase of the soluble phase from 55.1±2.7% to 97.0±3.3% which corresponds to a decrease in pH from pH 6 to pH 2. For the fortification of the milk, five calcium salts were selected for synthesis. The salts were calcium carbonate, tricalcium di-citrate, calcium gluconate, calcium lactate and tri-calcium phosphate. The fortification was performed with all of the synthesized salts except calcium lactate and tri-calcium phosphate. The starting material for the syntheses, calcium chloride, was also used for fortification. It was found that for the fortification of CaCO3 under gastric conditions a bioavailability of 100% could be achieved. However the acidic environment also resulted in coagulation of the milk and therefore the bioavailability could not be accurately assessed. Calcium gluconate had the only increase in bioavailability under normal conditions. In conclusion a better understanding of fortification with calcium salts was achieved and the influence of pH on the bioavailability of calcium in milk and the effects on the salt fortification.