Alkali metals are well known as promoters for catalytic reactions on transition metals. We have characterized coadsorbed CO and K on Ir(lll) by means of temperature programmed desorption (TPD) and Fourier transform infrared reflection-absorption spectroscopy. Carbon monoxide is known to adsorb at on-top sites on Ir(lll), and is characterized by a stretching frequency that varies from 2028 cm(-1) in the limit of zero coverage to 2090 cm(-1) at 0.7 monolayer (ML) which is saturation coverage on this surface. In the presence of K, however, several CO stretching bands with frequencies as low as 1395 cm(-1) were found in the IR spectra depending on the K coverage. CO and K desorb-simultaneously in a peak around 710 K with a half-width of 60 It, suggesting the formation of K-n(CO)(m) compounds for K coverages greater than 0.1 ML. In contrast to clean Ir(111), the adsorption of molecular hydrogen on the K precovered surface is activated. By exposing atomic hydrogen to this surface, the barrier for adsorption is overcome, and the hydrogen adatoms are stabilized by K, as shown by TPD. However, when atomic hydrogen is dosed on the K/Ir(lll) surface precovered with CO, the K-n(CO)(m), compounds that are formed inhibit the adsorption of even atomic hydrogen.