We describe the calibration of a Paris-Edinburgh press cell assembly, centred around a single-crystal diamond sample capsule. The combination of low X-ray absorbance and diffraction, limited reaction with the sample material, and extreme incompressibility preserving the sample geometry makes single-crystal diamonds ideal for in situ measurements of liquid properties such as density, viscosity, and short-range structure. Here, we use in situ synchrotron X-ray diffraction measurements at pressures up to 5.5 GPa and temperatures up to 2000K to show that diamond incompressibility significantly impairs pressure transmission to sample materials at low temperature. At room temperature, diffraction patterns of pressure calibrants situated inside the diamond sample capsule show that sample pressure remains constant at ∼0.3GPa, even when the same pressure calibrants positioned outside the capsule show assembly pressure reaches up to ∼3.0GPa. During heating, inside and outside pressures converge, with full pressure transmission achieved at 1150-1250K. Our results demonstrate that diamond sample capsules are suitable for property determinations of most anhydrous silicate and iron-rich metal melts, but that sample pressures in low-temperature experiments (e.g. focusing on properties of hydrous fluids and melts) could be significantly overestimated. © 2010 Taylor & Francis.