The variation of stress with respect to temperature in ZnO films, prepared by r.f. magnetron sputtering on Si and GaAs substrates, has been studied by using a bending beam technique. The thermal expansion coefficients and biaxial moduli of the films have been determined from the stress-temperature curves upon cooling. For most of the films, the obtained biaxial modulus ranges from 250 to 360 GPa at temperatures from 25 to 400 degrees C, whilst the thermal expansion coefficient increases from 5 x 10(-6) to 8 x 10(-6) degrees C-1. X-ray diffraction results reveal that the films deposited at a higher total gas pressure, i.e. 5.3 Pa, or a lower substrate temperature, i.e. 250 degrees C, have further crystallized upon annealing. The crystallization has caused the films to exhibit an irreversible stress behaviour upon thermal cycling.