Investigations of the structure of polypropylene fibres coloured with a mixture of quinacridone and phthalocyanine pigments were carried out. The structure of the fibres, evaluated by wide-angle X-ray scattering and small-angle X-ray scattering methods, was compared to the structure of noncoloured fibres and the fibres coloured with particular pigments. It was stated, that pigments participate in the nucleation process at low take-up velocity. In the presence of pigments, nuclei of two polymorphic forms alpha and beta are formed. In fibres coloured with the mixture of pigments the competition between formation of the alpha and beta nuclei occurs. The beta nuclei formed on the crystals of the quinacridone pigment dominate. The high number of the beta nuclei, together with the higher growth rate of the beta crystals, leads to the formation of a structure containing a large amount of the beta form, only slightly lower in comparison to the fibres coloured with the quinacridone pigment. At higher take-up velocities under higher molecular orientation in the cooled stream of the fibre, the very effective row nuclei are produced. In the presence of such nuclei, pigments do not participate in the nucleation process and do not affect the fibre structure. (C) 2003 Elsevier Ltd. All rights reserved.