Blends of bisphenol-A polycarbonate and a thermoplastic polyester (Kodar) are miscible as shown by single DSC and DMTA glass transition temperatures at all compositions. Blends of polycarbonate and Kodar also display a strong, single dielectric alpha -relaxation. Conclusions regarding the molecular nature of the blends, the state of mixing and molecular cooperativity can be made by both quantitative and semi-empirical analysis of dielectric relaxation data. There is an increased broadening of the loss spectra on blending as determined by both Havriliak-Negami (HN) and Kohlrausch-Williams-Watts (KWW) fits to the dielectric data, and by analysis of the DMTA loss peaks. An increase of the relaxation time on blending is attributed to the temperature of measurement being below the glass transition of the rigid PC-rich component of the blends and that this is likely to have an effect because of micro-heterogeneity in the blends. Local regions of composition rich in Kodar or polycarbonate exist in these blends and this allows the molecular motion of the PC molecules to dominate the relaxation behaviour of the blends. Concentration fluctuation is thus the dominant feature controlling the dielectric response in these miscible, amorphous blends. (C) 2001 Elsevier Science Ltd. All rights reserved.