"The paper deals with one of the important directions of modern materials science, namely the development and creation of new materials suitable for use in real electrochemical devices including components of solid oxide fuel cells such as electrolytic materials. In this regard, there is the task of finding inexpensive and technological solid electrolyte with high conductivity and stability in oxidizing and reducing atmosphere. Currently, promising devices are a medium temperature SOFC (200-500 C). Medium temperatures are the most optimal region in terms of energy costs. Promising ionic and protonic conductors for this temperature range are electrolytes based on the oxygen deficient complex oxides. The paper focuses on barium-calcium niobate which is well known oxygen-ion and proton conductor. The cationic substitution is the most common way for improving their transport properties. The anionic substitution is the new method which leads to increase the ionic and proton conductivity. Thus, the example of the proton conductor barium indium shows the principle possibility of substituting the fluoride and chloride ions for oxygen ions, that was accompanied by an improvement of the transport properties of halogenated phases with a low dopant concentration relative to the undoped matrix. In this paper, the novel type of protonic electrolytes - chlorine-substituted barium-calcium niobate Ba2CaNbO5.5-δ/2Cδ - was obtained for the first time. The synthesis was carried out by a ceramic method with a maximum annealing temperature of 1300 °C. It was found that the complex oxide obtained was characterized by the cubic structure of a double perovskite. The introduction of chloride ions, which have a large radius relative to oxygen ions, leads to an increase of the unit cell parameter. It is proved that the composition Ba2CaNbO5.5-δ/2Clδ was capable of dissociative absorption of water from the gas phase; the water uptake was lowered compares with the matrix composition Ba2CaNbO5."