We have investigated the valence electronic states of alpha-sexithiophene (alpha-6T) on three passivated Si(0 0 1) surfaces, oxidized Si(0 01), water-adsorbed Si(0 01) and ethylene-adsorbed Si(0 01), using ultraviolet photoelectron spectroscopy (UPS). At a thickness of a-6T layer below 0.5 nm, clear features of the pi states are observed for water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 01), whereas broad features are observed for oxidized Si( 0 01). This difference is attributed to the formation of well-ordered stacking on water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 01). (C) 2014 Elsevier B.V. All rights reserved.