An extensive metal compatibility study has been undertaken in order to qualify the high density cesium acetate brine for duty as a well construction fluid for use under ultra/extreme High Pressure High Temperature (HPHT) conditions. As a first step to qualify this new brine system, tests were conducted on metallic materials commonly used in downhole applications. These materials were tested for extended time periods at temperatures in the upper range of the material’s operational envelope. The fluid has been tested for Stress Corrosion Cracking (SCC) of martensitic, duplex, and nickel alloy steels up to temperatures of 2320C (4500F). Acid gases, such as CO2 and H2S, and air have also been added. Exposure time has been up to 6 months. Additional testing of duplex and nickel alloys has been conducted to check for hydrogen charging. The test results are very promising. Compared to other brines, cesium acetate seem to perform better in SCC tests, and hydrogen charging seem to be limited to very high temperatures and where the pH buffer has been overwhelmed. Based on the current test results, cesium acetate appears to be a good candidate fluid for ultra/extreme HPHT conditions. For new and less commonly used materials, more testing will be required to qualify the fluid for these specifically.