The effect of potassium on the reaction pathways of adsorbed CH2 and C2H5 species on Rh(111) was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TDS). Hydrocarbon fragments were produced by thermal and photo-induced dissociation of the corresponding iodo compounds. Potassium adatoms markedly stabilized the adsorbed CH2 and converted it into C2H4, the formation of which was not observed for K-free Rh(111). New routes of the surface reactions of C2H5 have been also opened in the presence of potassium, namely its transformation into butane and butene.