Interdiffusion was studied in Zn1-xMgxO:Al/ZnO expitaxial superlattices with periods of 15 and 25 nm grown on basal-plane sapphire by dc reactive sputtering. Interdiffusion coefficients were determined by analyzing low angle x-ray reflectivity peak intensity decreases during isothermal annealing, using analytical interdiffusion expressions. The results were corroborated with diffusion data obtained from secondary ion mass spectrometry compositional depth profiles. An initial fast diffusion stage was observed for, e.g., the first few hours at 500 degrees C, followed by slower diffusion at longer times.