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Results obtained via small-angle neutron scattering studies of the influence of calcium ions on the structure and phase transitions of phospholipid membranes are presented. The main phase transition temperature of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (1 wt %) multilamellar vesicles is demonstrated to increase by more than 1°C even when the calcium-ion content of the solution is low (0.1 mM). Detailed analysis of the multilamellar vesicles transition between “bound” and “unbound” state indicates the continuous character of the investigated process in both liquid and gel phases. The critical Ca²⁺ ion concentrations which initiate the destruction of the multilamellar structures and the formation of unilamellar vesicles are found to be ~0.3 mM in the gel and ~0.4–0.5 mM in the liquid-crystal phases during heating and ~0.5 mM in the phases under study upon cooling.