We report measurements of the viscosity of liquid di-isodecyl phthalate (DIDP) and a correlation determined therefrom. The viscosity of DIDP was measured at temperatures between (298.15 and 373.15) K and pressures between (0.1 and 140) MPa with a relative uncertainty of 2% by means of a new vibrating-wire viscometer. Additional measurements were made at a constant pressure of 0.1 MPa in the temperature range (274.07 to 368.21) K by means of capillary viscometers, with a relative uncertainty of 1 %. The results have been represented by a correlating equation containing six adjustable parameters with an average absolute relative deviation of 0.9 % and a maximum absolute relative deviation of 2.9 %. The available literature data are found to be generally in good agreement with our correlation at temperatures between (293 and 373) K at pressures up to 140 MPa, even when the corresponding viscosity is up to a factor of 10 greater than the largest viscosity measured in this work. A two-parameter correlation for the viscosity of DIDP at p = 0.1 MPa is also presented that represents the present results and selected data from the literature with an absolute relative deviation of 0.5 % and a maximum absolute relative deviation of 1.7 % at temperatures of (273 to 369) K. Viscosities of 1-methylnaphthalene at T = 298.15 K and pressures up to 140 MPa were also measured for purposes of verification.