A pressure-transmitting medium (PTM) plays an important role in diamond anvil cell (DAC) experiments as it ensures that the sample is exposed to hydro-static pressure. Although PTMs that are liquids under ambient conditions are the easiest to handle and load, the selection of chemically inert liquid media with established hydro-static properties is limited. To widen the choice of highly inert PTMs for high-pressure experiments, the hydro-static behaviours of Fomblin Z60, Fomblin Z25, Fomblin Y LVAC 06/6 and Halocarbon Oil 11-14 were investigated. The ruby fluorescence method was used to monitor the evolution of pressure gradients across the DAC sample chamber during compression and decompression. Fomblin Z60 and Fomblin Z25 perfluoro-polyethers, which are hydro-static to 1.7 and 1.5 GPa, respectively, exhibited the best hydro-static performance, followed by Halocarbon Oil 11-14 with a limit of 1.2 GPa, whereas the non-hydro-static behaviour of Fomblin Y LVAC 06/6 was observed above only 0.6 GPa.