The apparent diffusion coefficients Da of cationic dyes in polyacrylonitrile fibers with sulfonate sites was found to increase with increasing site concentration. This could not be attributed to a concentration dependence of Da. The rather minor plasticization by the sites themselves, or by water taken up by them, which could be detected between the extremes in site content, also did not appear to account for the strong dependence ob scrved. The rate of dye uptake was found to be almost independent of salt and hydro gen ion concentration in solution. The findings could best be explained on the basis of a diffusion mechanism in which dye penetrates by interchange between mobile occupied sites and unoccupied ones further inside the fiber. Fick's law was derived from the second-order kinetics of such interchange, with a diffusion coefficient proportional to the site concentration. Alternative rubber-like transport mechanisms, less in line with the present experimental data, are also presented.