Vegard's law and inductively coupled plasma atomic emission spectrometry were employed to determine the Mg composition in MgxZn1-xO layers deposited on 6H-SiC substrates. With the increase of Mg composition in MgxZn1-xO layers, the c-axis length decreased by 5.2048-0.072x, while the a-axis length increased to 3.2491 + 0.047x. The lattice constants estimated by Vegard's law and a theoretical model exhibited an uncertainty of similar to 3 % that has been attributed to the similar to 2 % lattice misfit in the MgO/ZnO materials system. Localized exciton peaks of MgxZn1-xO alloy in photoluminescence (PL) measurements disappeared completely, while the neutral donor-acceptor pair and 1-longitudinal optical-phonon energies decreased rapidly with the increase of Mg composition. These PL data do not comply with Vegard's law. The asymmetric behavior in the MgxZn1-xO alloy is the subject of locally disordered Mg potential fluctuations and an artifact of the C-MgO and a(MgO) lengths calculated theoretically. (c) 2005 American Vacuum Society.