The kinetics of the diffusion and the carrier action by the "loosening-of-fiber structure theory" in polyethylene terphthalate fiber dyeing with carrier have been investigated in the admixture of disperse dyes used in previous work. It has been found that carrier action and molecular area of disperse dyes, together with the free volume of dyeing substrate, may also well be inter-related with the diffusion behaviour of dyes in the admixture with carrier. The diffusion behaviour of same applicable component dye, however, has not been agreed with that of the component without carrier.rier.