Measured data for the dielectric constant of water exist to 550°C and 5000 bar and to 400°C and 3000 bar (critical temperature: 374°C). The present calculations extend to 1000°C and a density of 1 g cm−3 (16000 bar). Results are given for densities between 0.1 and 1 g cm−3 and for temperatures to 1000°C. From 400 to 1000°C at densities of 0.1, 0.5 and 1.0 g cm−3 the dielectric constant varies from 2.04 to 1.50, from 10.45 to 4.69 and from 29.4 to 11.06 respectively. The calculation is based on a hard‐sphere with dipole moment molecular interaction model and an approximate equation of Patey et al. based on a linearized hypernetted chain procedure. The effective hard sphere diameter of 2.68 Å has been derived from fitting to existing PVT data to 1000°C by a suitable equation of state. The effective dipole moment of 2.33 D was obtained from fitting to existing experimental dielectric constant values to 550°C and high densities with the Patey equation. Analogous calculations of the dielectric constants of supercritical, dense HCl, NH3 and CH3F are presented.

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A high pressure apparatus is described to measure simultaneously within one cell the dielectric constant and the density of non‐polar supercritical fluids to 400 °C and 3000 bar. Thus relations between the dielectric constant and the fluid density at any given temperature can be discussed with particular reliability. The apparatus consists of a cylindrical autoclave, which encloses a stainless steel belows containing the sample. A cylindrical capacitor (ca. 60 pF) is mounted inside the bellows. A back‐up fluid of heptane is used to produce the pressure on the bellows. Measurements are made between 10 kHz and 1 MHz. Values of the dielectric constant ϵ are reported for 10 temperatures between 27.5 and 400 °C and for pressures between 1 bar and 3000 bar. At 27.5°C and 1 bar ϵ is 2.273. At 100°C it varies from 2.124 to 2.425 between 1 and 2800 bar. At 400°C and 3000 bar ϵ = 2.198. The probable error is below 0.2%. Density measurements cover the same range. The temperature dependence of ϵ at constant density is very small. The Clausius‐Mossotti‐function is presented for the whole region. In accordance with existing theories it can be described as a three‐term series in powers of density and a T−1‐term.