An expression of the thermal diffuse scattering intensity based on the correlations among the thermal displacements of atoms has been newly given and is applied to a background function in Rietveld refinements. Neutron scattering measurements from powder samples of CuI at 290 and 8 K are performed. Rietveld refinements by the newly proposed background function give somewhat lower R factors at 290 K than the previous one. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.