The design and operation of a new vibrating-wire viscometer for the measurement of high-viscosity liquids (50 to 125) mPa.s, and up to 18 MPa are described. The design of the instrument is based on a complete theory so that it is possible to make absolute measurements. The viscosity of liquid diisodecyl phthalate (DIDP) is measured at temperatures between (293 and 363) K and pressures between (0.1 and 18) MPa with an absolute uncertainty of +/- 1.5 %.