A density functional study of substitutional carbon impurities in ZnO-host matrix with 6.25 at. % was performed applying both the supercell method and the coherent potential approximation. The first of these methods assumes the spatially ordered arrangement of carbon impurities in oxygen sites, whereas the second one simulates the spatially disordered impurity substitution of oxygen sublattice. In conjunction with a theoretical part, the X-ray photoelectron spectra of pure and C-doped zinc oxide were measured. The mapping of the valence band of the carbon-loaded ZnO allows establishing the appearance of impurity C 2p electronic states on the valence band top. The experimental energy position of these impurity C 2p states is theoretically reproducing essentially better by the assumption about random distribution of carbon impurities in oxygen sublattice.