Safety, lightweight design, and reduction of emissions are terms which are key issues in modern vehicle construction. These challenges can be met by new lightweight design strategies, e.g., by using lightweight materials and high-strength steels as well as innovative forming technologies such as media based press hardening (MBPH). MBPH as a sub-production technique of hydroforming is a tempered internal high-pressure forming process of closed profiles, which this article is about, or sheet metals by gaseous media. Due to the high process requirements (internal pressure up to 70 MPa and temperatures up to 1000 °C), it has not been possible to measure the temperature curve of the active medium in a reliable way until now. The aim of the research project described in this article was to develop an innovative measuring instrument to determine the gas temperature curve with a measuring frequency of at least 1 Hz. Analytical and numerical calculations have indicated that the active medium has a significant influence on the thermodynamic of the forming process. The finite element analysis (FEA) of the heat flow during the forming process has indicated that the influence of the gas on the cooling process of the work piece is about 15% of the total influence of the tool. Consequently, the active medium in media based press hardening processes is an important thermal influencing factor. Experiments have confirmed that it is possible to determine the calculated curve of the gas temperature and maximum temperatures of the active media up to 500 °C. The findings of these studies make a significant contribution to identifying and analyzing the complete temperature balance in tempered active media based forming processes.