We have measured the viscosity and density of certified reference materials N10 and S20 with nominal viscosities at T = 298 K and p = 0.1 MPa of (16 and 29) mPa center dot s, respectively, at temperatures in the range of (298 to 393) K and pressures between (0.1 and 55) MPa. These measurements extend the pressure range by 55 MPa and the temperature range by 20 K over which the viscosity and density of these fluids are known. Both a vibrating wire, with a wire diameter of 0.1 mm, and oscillating sinker viscometers were used for the measurements with standard uncertainties (k = 1) of +/- 1 % and +/- 2 %, respectively, whereas density was obtained from a vibrating tube densimeter with an uncertainty of +/- 0.1 %. The viscosity obtained from both instruments agreed within a reasonable multiple of the combined expanded uncertainty over the whole range of viscosities from (1.8 to 76) mPa center dot s, where the latter value represents the upper useful operating viscosity of the vibrating wire. The measured viscosity and density have been combined with the appropriate certified values for N10 and S20 atp = 101.325 kPa to obtain interpolation expressions with leading terms based on the cited values. Our equation represents the measured viscosities to within +/- 2 % and the densities to within +/- 0.2 % that are consistent with the expanded uncertainty (k = 2) of our measurements.