The CO adsorption on Cs-covered Ru(001) surfaces has been studied by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). When CO molecules adsorb on the Cs-preadsorbed surfaces at 85 K, two-dimensional Cs + CO islands are formed for Cs coverages lower than 0.25, which exhibit p(2 x 2) LEED patterns irrespective of both Cs and CO coverages. The p(2 x 2) Cs + CO island has an almost constant [CO]:[Cs] stoichiometry of 2:1 at Cs coverages below 0.2. This stoichiometry changes to 4:3 when the islands cover the whole surface (theta(Cs) = 0.25). The local density of Cs in the island is 0.25 independent of Cs coverage. The C-O stretching frequency of the CO species located outside the island shifts from 2075 to 2000 cm(-1) in proportion to Ca coverage. The C-O stretching mode of the inside CO species appears at much lower frequencies (1800-1580 cm(-1)), which depend on the [CO]:[Cs] ratio. The former and the latter frequency shifts are associated with a weak long-range and a strong short-range effect of Cs, respectively. The isotope exchange between the isotope-labeled CO species occurs exclusively inside the islands. The temperature onset of the isotope-exchange reaction is estimated to be 450-500 K. This is comparative to the temperature onset of the desorption from the 2:1 state, while it is lower by 100 K than that from the 4:3 state, which results in a distinct difference in the exchange fraction for these states. The formation mechanism of the p(2 x 2) Cs + CO island is discussed based on the recently proposed models of the alkali + CO coadsorption process.