Chemical Vapour Deposition (CVD) is a well-established process for the deposition of polycrystalline diamond films on silicon substrates. However, for highly demanding applications such as biomedical components the diamond film surface is too rough. In order to decrease the crystal size and reduce surface roughness, inert gases such as argon and helium can be added to the CH4/H2 precursor gases. In this study we present the thermodynamic aspects of the gas phase chemistry involved in hot filament CVD of argon/CH4/H2 and helium/CH4/H2 systems used to grow smooth nanocrystalline diamond films. Experimental results on the growth, surface morphology and crystalline structure are also presented. It is evident that the addition of argon and helium has a considerable effect on the gas surface chemistry and the ratio of the methyl and acetylene ratio has a significant effect. These in turn influence surface morphology and crystallinity.