Abstract We present the first measurements of carbonate melt viscosity and density at mantle pressures and temperatures and provide important data for modelling carbonatite behaviour within the mantle. Synchrotron radiation was used to observe falling spheres with high atomic number in situ, allowing precise determination of high terminal velocities over short fall distances. The measured viscosities of 1.5 (5) × 10 −2 to 5 (2.5) × 10 −3 pas are the lowest of any known terrestrial magma types and these measurements extend the region of measurable viscosity at high pressure by at least 2 orders of magnitude. Accurate measurements of K 2Ca(CO 3) 2 melt density were performed at atmospheric pressure: ϱ(g/cm 3) = 2.39(2) − 3.85(15) × 10 −4 T (°C) and are in complete agreement with the predictions of Wolff (1995). The high-pressure density measurements also agree well with molecular dynamics predictions of carbonate melt compressibility.