The constant photocurrent method was originally developed to determine the sub-bandgap absorption coefficient of hydrogenated amorphous silicon. We studied the usefulness of this method for conjugated polymers and blends of polymers and fullerenes. More precisely, the polymer poly [2-methoxy-5- [2'-ethyl-hexyloxy]-1,4-phenylene vinylene] (MEH-PPV), the fullerene derivative (6,6) phenyl C61 butyric acid methylester PCBM and blends of MEH-PPV and PCBM were investigated. These blends are of great interest for photovoltaic applications because of the efficient generation of photocarriers. This is a consequence of the charge transfer process that occurs between the polymer, which acts as a donor and the acceptor-like fullerene. (C) 2003 Elsevier B.V All rights reserved.