Single-phase wurtzite Zn1-xMgxO alloy films with 0 ≤ x ≤ 0.45 were successfully grown on (111)-oriented Si substrates by molecular beam epitaxy. Although the Zn1-xMgxO alloy films with x > 0.3 exceeded the solid solubility limit at 600° C, the growth at lower temperatures was found to be effective to raise the limit up to x = 0.45. Both energies of the cathodoluminescence peak and optical absorption edge showed continuous blueshifts with increasing the x value till the solubility limit, although the shift of the former energy became shorter than that of the latter one presumably due to the spatial inhomogeneity in alloy films. Photoresponse measurement for Zn1-xMgxO/Si photoconduction cells revealed visible-blind characteristics with specific cutoff wavelengths in accordance with relevant bandgap energies to their x values. © 2005 Elsevier B.V. All rights reserved.