Potassium promoted niobium oxynitride catalysts were investigated by species resolved thermal alkali desorption (SR-TAD) method, Energetic barriers for potassium ions and atoms desorbing from the catalyst surface were determined. They are in the range of 1.0-2.3 and 2.3-3.0 eV for K and K+, respectively, depending on the way of the promoter introduction (via nascent- or post-doping). The work function (3.0-3.8 eV) was evaluated from the Schottky energy cycle. The shifts of the Fermi edge upon K doping were confirmed by parallel XPS studies. The surface stability of the promoter and its electronic action were found to be inimical. The lowering of the work function was greater for the post-doped sample, but the stability of potassium was distinctly lower than that observed in the case of the nascent one.