In order to examine x-ray absorption near-edge structures (XANES) of disordered solid solutions of metal oxides, a combined approach of a first principles supercell method and a cluster expansion method is developed. Zn- L3 edge XANES are measured on a series of Mg1-x Znx O with a rocksalt structure in the range of x=0.025-0.3 using synchrotron source. A first principles orthogonalized linear combinations of an atomic orbital method is employed to obtain a theoretical spectrum of a given model. A Zn-2p core hole is included in the calculation, and a set of 128 atom supercells is used. Theoretical XANES of disordered solid solutions are obtained as a weighted sum of theoretical spectra for four ordered structures, with the weighting factors determined by the cluster expansion method. The dependence of the spectral shape on the solute concentration is reproduced only when the averaged environment of solute atoms as determined by the solute concentration and the effect of the disordering is taken into account. The formation of the disordered Mg1-x Znx O solid solution is confirmed by the Monte Carlo calculations. © 2007 The American Physical Society.