The relative static dielectric constant ε of methyl cyanide was measured from 20 to 350°C, from 50 to 2250 bar and for the liquid along the saturation line to 270°C (Tc = 274.7°C). — Equipment and experimental procedure are described. At 20°C and pressures of 50 and 2000 bar ε changes from 37.7 to 42.9. At 100°C the change over the same pressure range is from 26.6 to 32.4 and at 300°C from 2.50 to 18.5. The accuracy of the data is ±0.4% up to 100°C and ±0.6% up to 300°C. Above 300°C and below 200 bar the data may be uncertain within ±1 or ±2%. Diagrams with ε‐values in dependence of density and temperature are given. The Kirkwood correlation factor below a density of 0.4 g cm−3 is slightly above unity. At densities higher than 0.4 g cm−3 ε declines at all temperatures to about 0.8 at 0.8 g cm−3. This may indicate a certain degree of dimerization, which is discussed.

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