Ultrathin potassium layers were deposited on hydrogen passivated Si(001)-1x1 surfaces at a temperature of 120 K in the thickness range from submonolayers up to 3 nm. They were investigated with Auger spectroscopy, work function, and current-voltage measurements. After the formation of a wetting layer, potassium deposition leads to island growth. The surface hydrogen atoms are removed by the adsorption process. By attaching an electrical contact to the thin film, the current-voltage characteristics of the Schottky diodes could be determined. The analysis yields a homogeneous Schottky barrier height of 0.55 eV for K/p-Si(001) diodes in agreement with the metal induced gap state model. Exposing the diodes to molecular oxygen generates a strong chemicurrent signal which first increases with exposure, passes a maximum, and drops exponentially. The chemicurrent transient is attributed to a nucleation and growth formation of oxide islands and gives strong evidence for the existence of precurser states prior to oxidation.