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The thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3 (LSAT), substrate materials for superconducting thin-film device applications was measured by Rietveld refinement of neutron powder diffraction data over the temperature range of 15-1200 K. Results indicate that both materials exhibit nearly the same relative volume thermal expansion but the cell volume for LSAT is slightly larger. The rms displacement of the A site in LSAT is significantly bigger than that for LaAlO3 and about half of the difference can be accounted for by a static displacement component.