The molecular diffusion coefficients of 2-nitroanisole, 1,2-dichlorobenzene and tert-butylbenzene in supercritical carbon dioxide and carbon dioxide containing modifiers were determined by using the Taylor-Aris dispersion technique. Experimental values are reported for temperatures ranging from 313 to 333 K and pressures between 1.5.0 and 35.0 MPa. The influence of pressure, temperature, density and viscosity on the binary diffusion coefficients was examined. The addition of low proportions of methanol and n-hexane as modifiers had an important effect on the diffusion coefficient of the three solutes. (C) 2002 Elsevier Science B.V. All rights reserved.