Adsorption isotherms, apparent diffusion coefficients, activation energies of diffusion and affinity have been determined in order to clarify the dyeing mechanism of cationic dyes on cationic dyeable polyester fiber. The dyeing mechanism is essentially governed by an ion-exchange equilibrium described by a Langmuir isotherm type, the related combination of cationic dyes and cationic dyeable polyester fibres should be an ionic bond. The apparent diffusion coefficients of cationic dyes on CD-PES fibres increase in proportion to the number of sulphonic acid groups due to the relaxation of the structure of CD-PES fibres. The values of activation energy are lower than those of acrylic fibres and regular polyester fibres, therefore, CD-PES fibres depend on the temperature in terms of the rate of dyeing less than other hydro-phobic synthetic fibres. The affinities increase in proportion to the number of sulphonic acid groups. The affinities of Dye I are higher than those of Dye II, which seems to be due to the chemical structure of dye.