- It has been reported recently that kinetic energy of photoelectrons emitted from core levels decreases with decreasing of the nanocrystal size. This phenomenon is called the size shift. The size shift value is the same for donor and acceptor in the compound. The present work is aimed on the explanation of this phenomenon. Crystals of lead sulfide PbS with different size from 50 to 350 nm were grown by chemical bath deposition (CBD) technique from alkaline solution onto Si and GaAs substrates. The morphology and size of crystals were analyzed by high resolution scanning electron microscopy (HRSEM). Complex electron spectroscopy investigations of electronic structure were carried out. In recent experiments X-ray photoelectron spectroscopy (XPS) was used for determination of Pb 4f, and S 2p electronic level positions and their size shifts. To explain the observed dependences in this work, we applied the following methods: analysis of PbS valence band (VB) and Pb 5d electronic level structure in the range ∼0–30 eV by XPS, high resolution electron energy losses spectroscopy (HREELS) for analysis of band gap transformations and work function measurements by Kelvin probe microscopy for the contact potential difference (CPD). The influence of work function increasing, widening of the band gap, transformations in VB and inter-level energy distances with decreasing of nanocrystal size on the size shift function ΔE(R) is discussed.