The optical band gap and the valence band of CdSe nanocrystals were investigated with respect to surface capping molecules by using photo luminescence and synchrotron photoemission spectroscopy. The optical band gap was shown to be mainly dependent on the size of the molecule controlling the inter-particle distance; the valence band, however, mainly depended on the chemical bonding nature of the molecule. UV in air and anneal under ultra high vacuum (UHV) were carried out to control the surface capping pyridine content on CdSe nanocrystal films. With UV treated film, a great reduction in the pyridine content was found, but extra oxygen was absorbed onto the naked CdSe surface. The surface absorbed oxygen exerted a strong influence on the electronic structure of CdSe nanocrystal films. This extra oxygen was successfully removed though UHV-anneal, and it was possible to distinguish the effect of oxygen from the effect of surface capping pyridine on the electronic structure of CdSe nanocrystal film. A quantitative relationship between the valence band maximum of CdSe nanocrystal films and the content of surface-absorbed oxygen could be given. (c) 2005 Elsevier B.V. All rights reserved.