The photophysical properties of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) are found to differ depending on its concentration in thin films. A film of dispersed PCBM molecules in polystyrene (PS) showed only weak absorption in the visible region (400-700 nm) with a single S1 → S0 emission band at around 700 nm. By comparison, a film of pristine PCBM showed heightened absorption in the visible region and the presence of an additional higher energy emission at ∼500 nm assigned to the presence of charge transfer (CT) transitions in crystalline PCBM. The energy of the CT exciton is estimated to be ∼2.8 eV. © 2007 Elsevier B.V. All rights reserved.