We report the thermal stability of room-temperature RF-sputtered Mg0.4Zn0.6O thin films and ZnO/Mg0.4Zn0.6O superlattices at 600 degrees C and 800 degrees C. The phase of room-temperature as-sputtered Mg0.4Zn0.6O is crystalline ZnO embedded in an amorphous or short-range-ordered hexagonal MgZnO matrix. Annealing at either 600 degrees C or 800 degrees C for 5 min transforms the matrix into a crystalline hexagonal wurtzite structure, leading to a decrease of the optical bandgap (E-g) of Mg0.4Zn0.6O. This also results in a slight change near the absorption edge of the superlattice transmission spectrum. The films precipitate cubic MgZnO after heating Mg0.4Zn0.6O at 800 degrees C for 5 min; by contrast, precipitations take at least 3 h if the samples are heated at 600 degrees C. Heating at 800 degrees C for more than 3 h significantly reduces the film thickness and E-g, attributed to the decomposition of superlattices and diffusion of magnesium into the substrate, respectively. On the other hand, annealing the ZnO/Mg0.4Zn0.6O superlattice at 600 degrees C for 12 h also produces an initial slight change in the optical transmission spectra, yet the spectra remain essentially unchanged for the remainder of the annealing process. (C) 2011 Elsevier B.V. All rights reserved.