Non-stoichiometric tin oxide thin films were deposited from an O-2/Ar/tetramethyltin (TMT) mixture in a RF glow discharge parallel plate reactor at low pressure (15 Pa) and low temperature (25-80 degreesC). The aim of this work was to determine the role of the experimental parameters such as the absorbed power, bias voltage and feed gas composition on the electron density and the atomic: oxygen mole fraction, two parameters which play an important role on the TMT decomposition and carbon stripping of the deposited films. Since TMT concentration is typically less than 2% a first study was carried out from both experimental and modelling point of view for the O-2-Ar mixture. Experimental investigations included optical emission spectroscopy, mass spectrometry, and electrical diagnostics. A numerical quasi-homogeneous (OD) model and a self-consistent ID fluid model were used for comparison. The optimisation of the thin film deposition process was discussed briefly on the basis of the obtained results.