Homoepitaxial MgxZn1-xO:P thin films with variable magnesium content (x < 0.05) have been grown by pulsed-laser deposition. The film compositions have been determined by proton induced x-ray analysis. The phosphorous and magnesium content of the films decreases with increasing oxygen partial pressure during growth. High-resolution x-ray diffraction shows a pseudomorphic growth of the thin films with a nominal MgO content of 2 wt %. Hall-effect measurements show electron mobilities of the as-grown films as high as 820 cm(2)/V s at 55 K, which compares to the value observed for ZnO:P homoepitaxial thin films. Upon annealing, the free carrier concentration was reduced by several orders of magnitude, depending on the oxygen partial pressure used during growth of the MgZnO:P films, while it remained nearly unchanged in ZnO:P samples. The role of the MgO concentration in the films in acceptor defect formation is discussed.