The approach of building biomolecular devices starts with the preparation of thin biomolecular films with a structural control. The first question regarding a biomolecular film is how many molecules are deposited on the substrate, or equivalently, what is the mass of the film. The optical methods, like ellipsometry and surface plasmon resonance (SPR), which describe the film in terms of experimentally measurable optical parameters, allow a precise determination of the thickness and the mass of the film. Other methods like quartz crystal microbalance (QCM) allow the determination of other parameters of the film, like viscosity and water content. A combination of these methods would allow a better characterization of the parameters of the biomolecular film.