Studying mass transfer phenomena in polymeric films subjected to impregnation with a supercritical fluid can give interesting information for textile dyeing performances and for applications in the pharmaceutical field. The aim of this work is to propose an experimental technique to evaluate diffusion coefficients of disperse dyes in a PET film. The 'film roll method' was used by other researchers in water impregnating systems to evaluate diffusion coefficients of non-ionic dyes in synthetic polymers. It has been extended and adapted to a supercritical impregnating system. Diffusion coefficients of two disperse dyes at different temperature (90 and 110 degrees C) and pressure values (22 and 25 MPa) have been measured and their dependence on working conditions gives a first explanation of mass transfer phenomena of disperse dyes in a PET film: experimental data can also be used to evaluate diffusion-concentration dependence. (C) 2000 Elsevier Science B.V. All rights reserved.