Broadband-IR sum-frequency generation spectroscopy was used to investigate the CO-stretch vibration of CO chemisorbed on a Ru(001) surface at coverages as low as 0.001 monolayers (ML). Due to the high intensity of the broadband-IR pulses the nu = 1 --> 2 hot band of the CO-stretch vibration of CO on pure and oxygen-covered Ruthenium is observed for the first time. The simultaneous detection of the fundamental and hot-band transition allows the determination of the anharmonicity constant x(e omega e) directly. For a coverage of 0.004 ML one obtains 13.6 cm(-1) for (CO)-C-12-O-16 and 14.5 cm(-1) for (CO)-C-12-O-16 co-adsorbed with O.