The methane decomposition in low pressure radiofrequency (13.56 MHz) leads to a growing film on the powered electrode and to a particle generation in the plasma bulk. The particles are negatively charged. During their formation, the electron energy distribution function and the electrical parameters of the discharge are modified. A small quantity of argon gas (5%) is introduced in the reactor in order to get information about the electronic state of the plasma. The aim of this study is the electron temperature determination. The plasma is analyzed by Optical Emission Spectroscopy. The electron temperature is calculated using the Corona balance model from, on one hand, the Hα, Hβ and Hγ Balmer line intensities, and on the other hand, from argon emission lines. The temporal evolutions of the electron temperatures during the particle formation obtained from hydrogen and argon intensities are compared and discussed.