The viscosity and density of diisodecyl phthalate (DIDP), with a nominal viscosity at T = 298 K and p = 0.1 MPa of 87 mPa center dot s, has been measured at temperatures from (298.15 to 423.15) K and pressures from (0.1 to 70) MPa. A vibrating wire viscometer, with a wire diameter of about 0.15 mm, was used for the viscosity measurements at pressures up to 70 MPa, and the results have an expanded uncertainty (k = 2), including the error arising from the pressure measurement, of between +/- (2 and 2.5) %. The density was obtained from two vibrating tube densimeters, one for operation at p approximate to 0.1 MPa with an expanded uncertainty (k = 2) of about +/- 0.1 % and the other one that functioned at pressures up to 70 MPa, with an expanded uncertainty (k = 2) of about +/- 0.3 %. Measurements of density and viscosity at p = 0.1 MPa were conducted with three samples of DIDP each with different purity stated by the supplier and as a function of water mass fraction in the range (20 to 417)center dot 10(-6). The values obtained agreed within the estimated expanded uncertainties of the measurements. One sample was from the same lot and purity as that used by both Caetano et al. (J. Chem. Eng. Data 2005, 50, 1875-1878) and Harris and Bair (J. Chem. Eng. Data 2007, 52, 272-278) for their measurements of viscosity and density. The measured viscosity and density are represented by interpolating expressions with differences between the experimental and calculated values that are comparable with the expanded (k = 2) uncertainties. The viscosities at p = 0.1 MPa agree with values reported in the literature within the combined estimated expanded (k = 2) uncertainties of the measurements while our densities differ by no more than +/- 0.15 %. At p > 0.1 MPa the only other literature values are those reported by Harris and Bair. Deviations of their values from our smoothing equation increase with increasing pressure to be < +/- 2 % at p < 1 MPa and between (-9 to 11) % at p = 50 MPa; these differences are within 2.5 times the combined uncertainty.