Nanocrystalline diamond (NCD) films in an amorphous matrix have been deposited by standard microwave plasma chemical vapour deposition from CH4/N-2 mixtures, and subsequently characterized comprehensively with respect to their structure and morphology, their composition. their crystalline properties, and their bonding environment. Thereafter, some of the coatings have been subjected to rapid thermal annealing (RTA) at 1100 and 1400 C, respectively. Characterization of the annealed films by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) revealed the loss of material upon annealing; while the bonding structure remained almost unchanged at least after the 1100 C treatment. RTA at 1400 C leads to the formation of crystalline SiCxNyOz material, very probably by reaction with the residual gas, whereas crystalline graphite is not observed. FTIR measurements indicate that hydrogen containing species from the residual gas may contribute to the loss of material. Finally, the morphological changes observed after the annealing give some insight into the growth mechanisms of these NCD films which are discussed in terms of a spherulitic growth caused by a rather low primary nucleation density but very high secondary nucleation rate. (C) 2004 Elsevier B.V. All rights reserved.