The effect of isothermal annealing on residual strain in the lattice of as-deposited basal crystallographic orientation zinc oxide was studied. Films were prepared by rf diode sputter deposition from a zinc oxide target and annealed in air for 5 to 1995 min at 300 or 500 °C. The change in crystallography as a result of annealing was determined by x-ray diffraction. The results show that as-deposited films have a residual tensile strain along the c axis with an average value of +0.62%. Strain relaxation begins to occur after 5 min at 300 °C. The small activation energy, ∼0.2 eV, suggests that interstitial diffusion assisted by local plastic strain may be the process by which strain relaxation initiates. Annealing at 500 °C for longer than 5 min causes a compressive strain to develop along the c axis upon cooling. This effect is attributed to the difference in the coefficient of thermal expansion of zinc oxide and silicon. Comparison with other annealing studies suggest that the magnitude of this effect is likely to be dependent upon the structure of the film–substrate interface.