Confocal laser scanning microscopy (CLSM) is used to monitor the dye penetration in poly(ethylene terephthalate) fibres during a common commercial dyeing process. The study of optical sections of fibres dyed for prolonged times in the dyeing process allows for a qualitative interpretation of the dye diffusion process, which is not possible by other techniques. A major dissimilarity was revealed between the diffusional behaviours of the two dye classes studied. The anthraquinone dyes showed a much lower temperature onset of dye penetration during the dyeing process than the benzodifuranone dyes (approximately 110degreesC vs. 130degreesC). Modulated differential scanning calorimetry was used to permit an understanding for this lower temperature onset and thus for the distinction in the driving forces of the dyeing mechanism between both dye classes. These driving forces were further shown to be similar for both the conventional textile fibres and the microfibres studied. (C) 2004 Elsevier Ltd. All rights reserved.