Stoichiometric compounds of the chemical formula Ba1.5-xSrxZr4P5SiO24, x being 0, 1.0, 1.25,and 1.5 have been synthesized by the ceramic procedure. The compounds show low thermal expansion behavior with x = 1.0 and 1.25 showing almost 0 thermal expansion up to 800degreesC. High temperature X-ray diffraction studies have been carried out on these samples and the axial thermal expansion coefficients calculated at each temperature by using the lattice parameters. The thermal expansion behavior is attributed to the unique crystal structure and the ionic substitutions of Ba and Sr in [M] site. (C) 2002 Elsevier Science Ltd. All rights reserved.