Photobleaching behavior of Cy5-conjugated bovine serum albumin (BSA) was investigated on the ensemble and single molecule level. For ensemble studies, a model was developed which accounts for different excitation rates of randomly oriented dye molecules. Both ensemble and single molecule observations yield non-exponential photobleaching dynamics of BSA-Cy5, indicating the existence of multiple bleaching pathways. (C) 2005 Elsevier B.V. All rights reserved.