During the growth of a diamond layer on a silicon substrate obtained by microwave plasma-assisted chemical vapour deposition, pyrometric observations show signal oscillations. We can prove that these variations are due to emissivity modifications linked with thickness increase. The purpose of the present study, in addition to its theoretical aspect concerning the conception of an emissivity model, is to show the possibility of using the emissivity observation (here a simple pyrometric measurement) as an in situ and real-time means of controlling the surface and therefore also the treatment process. The proposed model allows one at each moment, to calculate the thickness and the complex refractive index of the deposit, to measure its kinetic growth and therefore also to watch its possible evolution.