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Studies have been made of the dependence of saturation limits, rates of diffusion, rates of dyeing, and migration properties of a number of disperse dyes on polyester fibres upon the temperature at which the fibres have been heat-set. Use of pure, crystalline dyes was found to give erratic results and therefore ultra-dispersed commercial dyes were used in the investigation. The saturation limit and the average diffusion coefficient showed a minimum at a particular heat-setting temperature. Values were calculated for heat of dyeing, heat of solution, and entropy change in dyeing; these values tended to a maximum or minimum for fibre set at 150C. The findings suggest that the minimum tinctorial yields obtained at such temperatures are due to (a) increasing transformation of the fibre structure on heat setting and (b) preferential loosening of unset fibre by swelling agents in a heated medium. Degree of setting was found to have virtually no influence on migration. The causes of unlevel dyeings are discussed.