An investigation has been carried out of the strain and strain-relaxation in (111) oriented Pb films deposited onto oxidized Si substrates at 300 K and then cooled down to temperatures as low as 4.2 K. The strain in the Pb caused by differences in thermal expansion of the Pb and Si was measured using X-ray diffraction techniques. For film thicknesses of ~1400 Å, the magnitude of the observed strain at 4.2 K was found to approximate the strain value calculated from the difference in thermal expansion coefficients using the biaxial strain model. For greater film thicknesses, the value of the strain measured immediately after cooling was much lower, indicating that significant strain-relaxation occurred during the cooling process. It is probable that most of the strain was relaxed by a dislocation-slip mechanism. During isothermal annealing above 50 K a second, slow relaxation process was observed. Nonuniformity of the strain normal to the film-surface was found by analyzing the asymmetric X-ray line broadening that occurred during cooling. A strong dependence of strain on grain orientation was observed, and interpreted in terms of the biaxial strain model. © 1978.