Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM(3+)SnP(3)O(12) (A = Ca, Sr and Ba; M3+ = Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073 K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the 'sodium zirconium phosphate behaviour'. However, the strontium-and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds. (C) 2003 Elsevier Ltd. All rights reserved.