Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO2 interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.