We have measured the viscosity and density of certified reference material S20, with a nominal viscosity at T = 298 K and p = 0.1 MPa of 29 mPa·s, at temperatures in the range of (273 to 423) K and pressures between (0.1 and 275) MPa. A vibrating wire viscometer, with a wire diameter of about 0.15 mm, was used for the viscosity measurements at pressures up to 70 MPa, and the results have an expanded uncertainty (k = 2) of ± 2 %, while a falling sinker viscometer was used for measurements at pressures up to 275 MPa with an expanded uncertainty (k = 2) of ± 2.3 %. The density was obtained from vibrating tube densimeters with an uncertainty (k = 2) of about ± 0.2 %. The measured viscosity and density are represented by interpolation expressions. Our equation represents the measured viscosities to within ± 2.3 % and the densities to within ±0.2 %. These differences are comparable with the expanded uncertainty (k = 2) of our measurements. The measurements extend the pressure range by 275 MPa and the temperature range by 50 K over which the viscosity and density of these fluids are provided by the supplier. These measurements complement those reported in the literature for S20, at pressures and temperatures exceeding the certified values, and extend the temperature range by 30 K and the upper pressure by 220 MPa. The viscosities reported here differ from values reported in the literature for batches different to that used here by less than ± 4.5 %, which is within the combined estimated expanded (k = 2) uncertainties of the measurements and places a plausible bound on the certainty of η(T, p) for another batch of S20 that might be used as a calibrant for other instruments.