The thermal expansion of a lead sulfide nanofilm produced by chemical bath deposition was determined by X-ray diffraction (XRD). The thickness of the synthesized film was about 100 nm, and the average size of the coherent scattering regions as determined from XRD was about 40 nm. The lattice constant of the PbS nanofilm was measured as a function of the annealing temperature from 293 to 473 K and as a function of the annealing time at a constant temperature of 423K. The thermal expansion coefficient derived was found almost twice as large as that for coarse-grained PbS. (C) 2013 Elsevier B. V. All rights reserved.