Presence of a K adlayer will promote nitridation of GaAs(100) surfaces at room temperature, as is evidenced by photoemission measurements. Following exposure of K/GaAs to pure nitrogen, there appear remarkable variations in both valence band and core level spectra, i.e., recovery of work function to a certain extent, emergence of the N2p peak in the valence band spectra and creation of chemically shifted Ga2p and As2p components in the core level spectra. For the three different K coverages we studied (1/4, 1/2 and 1ML), the 1ML one shows the strongest promotion effect.