Gas composition is a fundamental parameter in the production of thin solid films by chemical vapour deposition (CVD), and it is especially critical in some advanced CVD processes such as low pressure CVD and atomic layer epitaxy (ALE). Because there do not exist on the market specific instruments for the measurement of gas composition in CVD plants, film producers are experiencing problems in controlling the level of contamination and in optimising the conditions of production. Our aim in the present project is to design and test a dedicated gas monitoring system based on Mass Spectrometry (lUS) and differential pumping in view of obtaining a system usable for routine monitoring of the CVD process, and at the same time in order to optimise the films quality by determining the functional properties (hardness, adherence, porosity, etc.) as a function of gas composition. This new instrument could lead to improved films, especially in some new production processes such as the micropulsed plasma enhanced chemical vapour deposition recently developed by one of the proposers (Rubig), as well as for some processes of high technological content, such as diamond and diamond-like films. This new technology, combined with the know-how gained on the influence of the process parameters on the functional properties of the films, would permit significant advances for the European industry in the strategically critical CVD manufacturing field.