The kinetics of the diffusion and the carrier action by the "loosening-of-fiber structure theory" in polyethylene terephthalate fiber dyeing have been investigated in the admixture of disperse dyes such as anthraquinone-azo dyes, axo-axo dyes, anthraquinone-diphenylamine dyes and anthraquinone-anthraquinone dyes systems. It has been found that carrier action and molecular area of disperse dyes, together with the free volume of dyeing substrate, may well be inter-related with the diffusion behaviour of dyes in the admixture. For the evaluation of carrier action of a component dye in admixture, diffusion coefficient and surface dye concentration in single dyeing systems were compared with those in binary mixture dyeing systems. Diffusion coefficient and surface dye concentration were obtained from the dyeings of single and mixture dyeing systems taken in the condition of infinite dye-bath and semi-infinite substrate.substrate.