We have determined the average preferred crystalline orientation of thin ultrananocrystalline diamond (UNCD) films using X-ray diffraction. The grain size and lattice parameters of the films were also calculated. We show how these characteristics change markedly with the gas chemistry used during growth, adding either 0-20% nitrogen or 0-15% hydrogen to the argon-rich, argon and methane microwave plasma used. We discuss how these changes give evidence that there is a competing growth mechanism between C-2 dimer mediated growth and the more widely used methyl radical growth process. Finally, we identify an additional X-ray diffraction peak, dependent on both the substrate used and growth conditions, as silicon carbide. We discuss these results in the context of the growth mechanisms of ultrananocrystalline diamond. (C) 2004 Elsevier B.V. All rights reserved.