Incorporation of low levels of fillers into thermoplastics can often lead to an increase in the modulus and a general improvement in the mechanical properties. However, increasing the level of filler loading can lead to a dramatic loss of impact strength and related mechanical properties. In this study the pigmentation of polycarbonate (PC) with titanium dioxide (TiO2) is shown to lead to a reduction in the fracture energy and changes in the failure mechanism of injection moulded test samples. Electron microscopic examination of the fracture surfaces indicates that changes in the failure mechanism are a result of the occurrence of large areas of densified polymer around pigment particles. Densification reduces the extent of segmental motion within the polymer matrix, as indicated by dielectric and thermally stimulated discharge current measurements. The influence on the molecular motion and mechanical properties of the degree of drying, molecular weight of the polymer and concentration of TiO2 are reported. Complementary positron annihilation measurements allow a tentative mechanism to be proposed to explain the loss of impact strength in these materials. Data is also presented on polybutylene terephthalate (PET) pigmented materials and a PC system in which the TiO2 has been mixed with polyethylene prior to dispersion in the PC matrix. These observations confirm the importance of polymer-pigment interactions in determining the fracture properties of samples.