Adsorption of CO on the Cu(110) surface modified with a low (near the work function minimum) potassium precoverage has been investigated by use of low energy electron diffraction, photoelectron spectroscopy based on synchrotron radiation, work function measurements (Delta Phi), and high resolution electron energy loss spectroscopy (HREELS). During potassium adsorption at 118 K the binding energy (BE) of the K 3p peak shifts -0.3 eV. For a thick potassium overlayer both a surface and a bulk K 3p peak, with a 0.9 eV difference in BE, are observed. The work function results at Theta(K) similar to 0.5 ML show after saturation with CO an increase of 1.11 eV. An electrostatic interaction energy between CO and K equal to 0.23 eV is determined. This energy indicates a moderate attractive interaction between CO and K. A CO overlayer on K/Cu(110) shows the 4 sigma satellite, the 4 sigma orbital, and the 5 sigma/1 pi joint feature. No observable change in the BE separation between the 4 sigma and the 5 sigma/1 pi band (3.1 eV) was found, thereby indicating a negligible CO-CO interaction. From the 4 sigma to the 5 sigma/1 pi intensity ratio (similar to 2) a weakening of the CO-metal binding with increasing CO coverage is deduced. Tn the HREEL spectrum, an intense feature at 1731 cm(-1) besides the fundamental stretching frequency at 2030 cm(-1) implies a substantial potassium induced weakening of the C-O bond.