Polymers with a small amount of dye blended in offer an attractive possibility to change the color of the emitted light by changing the dye. We present ab initio calculations within density-functional theory of the HOMO/LUMO energies for dipyrrolomethane dyes, polyphenylenevinylene and polyfluorene. Special attention is paid to the trends in these energies with variation of the sidegroups of the dyes as observed in cyclic-voltammetry measurements. From the energy-level alignments between dye and polymer we can understand and predict electron and hole trapping, crucial processes for the functioning of light-emitting devices based on these blends. (C) 2003 Elsevier B.V. All rights reserved.