Diffusion lengths for singlet states in PCBM films were estimated from analysis of annihilation rates in time-resolved emission studies. Results show the onset of singlet annihilation in PCBM films at absorbed photon densities above 1 x 10(18) per cm(3) and the singlet annihilation is well described by a time-independent value for the singlet annihilation rate constant (gamma) of 0.5 x 10(9) cm(3) s(-1). Limitations in our method means that our calculated value for the PCBM singlet diffusion length of 5 nm is likely to be a lower bound. (C) 2009 Elsevier B. V. All rights reserved.