High-temperature in situ X-ray diffraction was used to investigate isomorphism and the thermal expansion of apatite-structured compounds in three binary systems in the entire temperature range of the existence of its hexagonal modifications. Most of the studied compounds are highly expandable (alpha(1) > 8 x 10(6) (K-1)). In Pb-5(PO4)(3)F-Pb-5(PO4)(3)Cl system, volume thermal expansion coefficient is independence from the composition at 573 K. In Pb-5(PO4)(3)Cl-Pb-5(VO4)(3)Cl, the compound with equimolar ratio of substituted atoms has constant volume thermal expansion coefficient in temperature range 298-973 K. Ca-5(PO4)(3)Cl-Pb-5(PO4)(3)Cl system is characterized by the most thermal sensitive composition, in which there is an equal ratio of isomorphic substituted atoms. (C) 2014 Elsevier Ltd. All rights reserved.