By experience solar thermal systems are facing a lot of problems caused by gas enclosed in the hydraulic circuits (cavities or gas cushion). Problems known so far are flow instabilities as well as significant degradation of system efficiency. Solubility coefficients for relevant gases in solar liquids, in particular, are essential for in-depth theoretical analysis and can be presented for the first time. The measurements of the solubility coefficients of nitrogen in a typical solar fluid show a significant different behavior compared to water. The concentration of atmospheric gases was investigated in seven current large scale solar thermal systems by sampling. An indoor test rig was built and investigated in terms of solubility, corrosion and gas transport. Thus, this project presents both current data for existing solar thermal installations and also detailed results of the analysis of a system with well-controlled conditions.