The paper reports our first measurements of the viscosity of di-isodecylphthalate, which is a candidate for a reference material. At the same time it has a viscosity, which, at room temperature, is around 120 mPa · s, so that it can fulfill the need for a reference material more nearly matched to the needs of industry. The present measurements were carried out with a specially designed vibrating-wire viscometer over the temperature range 288–308 K and have an estimated uncertainty smaller than ±1.5%, following calibration against the viscosity of toluene. The instrument and results are presented here to encourage other measurements on the same material, by different techniques, which will lead eventually to the establishment of di-isodecylphthalate as a suitable reference material, as well as reference values for its viscosity.