Oxide pyrochlores of the composition Ln2M2O7 (Ln = La – Gd; M = Zr, Hf) have been prepared and their thermal expansion coefficients determined in the temperature range 298 – 1500 K by means of high temperature x-ray powder diffractometry. It is seen that for a given M atom, the expansion coefficient increases with the atomic number of the lanthanide element. Between the Zr and Hf series of compounds, the zirconates are found to be more expansive. The results support the ionic model for this class of compounds. © 1994.