The accuracy of the dissolved gas analysis is influenced by many factors, including diffusion, gas properties, oil sampling, etc. The gas properties, as well as the solubility of the gases, are greatly influenced by temperature, pressure, oil conditions, and type of insulating oil. Dissolved gases in transformers move in the insulating liquids through diffusion, natural temperature-dependent oil circulations and forced convection. In a system, dissolved gases tend to reach an equilibrium state throughout the entire system. Therefore, in free-breathing transformers, gas losses via the conservator tank can occur. When an equilibrium state is reached between the gas and the liquid phases, the correlation between the concentration of the dissolved gases in the liquid and the partial pressure in the gas phase can be described using Henry's law. The Henry constants are strongly dependent on the temperature and can only be determined experimentally. For this purpose, a state of the art test setup was developed and build up. The Henry constants are determined at different temperatures from 25 °C up to 90 °C for two combustible gases. The measured values are compared with values given by IEEE C57.104, IEC 60567, ASTM D-2779 and other research.