Pyrometric interferometry technique is carried out using a bichromatic infrared pyrometer in order to in-situ investigate the influence of the surface temperature variations on the properties of nanocrystalline diamond films, elaborated in Ar/H-2/CH4 microwave discharges for different gas compositions. The analysis of the film/substrate system apparent temperature measured by the pyrometer during the synthesis process indicates that the optical absorption of the growing film increases as the deposition temperature is raised. In particular, this trend is underlined by the increase of the film extinction coefficient derived from the experimental curves that ranges from 0.1-0.2 at low and moderate temperatures to 0.6-0.7 at high temperatures. This suggests that the formation of non-diamond absorbent phases within the film, especially sp(2)-hybridized compounds, is enhanced at high temperatures. This phenomenon is confirmed by further ex-situ characterizations giving evidence for a significant increase with surface temperature of the graphitic contents in the films. This demonstrates that the pyrometric interferometry is a powerful technique suitable for in-situ controlling and investigating the nanocrystalline diamond growth process. (c) 2004 Elsevier B.V. All rights reserved.