Optical emission spectroscopy (OES) was used to study the plasma generated by the activation of the gas phase CH4 + H, both by hot filaments and by a plasma discharge (DC HF CVD) during the nucleation of CVD diamond. The effects of nucleation parameters, such as methane concentration and extraction potential, on the plasma chemistry near the surface were investigated. The density of the diamond nucleation and the quality of the diamond films were studied by scanning electron microscopy (SEM) and Raman scattering, respectively. The OES results showed that the methane concentration influenced strongly the intensity ratio of H-beta-H-alpha implying an increase of electron mean energy, as well as CH, CH+, C-2. A Correlation between the relative increase of CH+ and the diamond nucleation density was found, conversely the increase of C, contributed to the introduction of defects in the diamond nuclei. (c) 2006 Elsevier B.V. All rights reserved.