The aim of the work was to investigate the effect of methanol addition on the dyeing of a commercial polyethylene terephthalate (PET) yarn with supercritical carbon dioxide. The solubility in the supercritical bath and the equilibrium uptake in the yarn have been measured for two azo dyes at different working conditions (353.2-393.2 K, 16-24 MPa). The partition coefficients between the phases were evaluated and compared with the data obtained without any modifier. It was found that the presence of methanol increases the dye concentration in the yarn and the dye solubility in the supercritical phase, and allows good dyeing results to be obtained at less severe working conditions. Both the dye solubilities and the equilibrium partition coefficients between the phases were satisfactorily correlated with some density-based equations available in literature. (C) 2008 Elsevier B.V. All rights reserved.