The initial growth of diamond films in a microwave plasma reactor has been studied usingin situtwo-color infrared pyrometry. Analysis of the observed oscillations of the apparent temperature has yielded the substrate temperature and also the instantaneous film growth rate and rms surface roughness &sgr;. Two distinct regimes of growth have been clearly identified: an initial period of rapidly increasing &sgr; before the diamond nuclei coalesce, followed by a slower increase of &sgr; with thickness as the continuous film grows further. The differing initial roughnesses and emissivities of Si and Mo substrates have been shown to have important effects on the growth of diamond. ©1998 American Institute of Physics.