We have studied the adsorption of acetylene on clean and K-covered Co(0001) by XPS, TDS and XPD. Acetylene is adsorbed molecularly at room temperature on clean Co(0001) with its molecular axis lying in the plane of the surface. The decomposition takes place at 410 K and goes likely via a vinylidene intermediate. Further heating results in the decomposition of the molecule to "graphitic" and "carbidic" carbon via the desorption of hydrogen. Potassium induces an additional acetylene adsorption state which is filled after the one found on clean Co(0001). The desorption of molecular acetylene and ethylene is induced by potassium. The desorption temperature of potassium is increased by 80 K due to acetylene when compared to the desorption of pure potassium monolayer. The decomposition of acetylene leads again to graphitic and carbidic carbon, but the relative amount of carbidic carbon is increased due to the presence of potassium.