There is an industrial requirement for a suitable reference material for viscosity measurements in the moderate to high viscosity region. Diisodecyl phthalate (DIDP) has been put forward as a candidate for this purpose. The field of elastohydrodynamics has a similar need for a reference lubricant that would allow the comparison of calculated contact behavior with experimental measurements. We report measurements of the viscosity of DIDP at atmospheric pressure between (0 and 100) degrees C and at high pressures to 1 GPa between (20 and 100) degrees C using three different falling body viscometers and samples from two different manufacturers. Densities obtained with a vibrating-tube densimeter are reported between (0 and 90) degrees C at atmospheric pressure.