Transient kinetic experiments allow to assess quantitatively the rate coefficients of individual elementary steps. Experiments were performed in the temperature range 300-400 K with temporal analysis of products (TAP) for CO oxidation over polycrystalline platinum, initially fully precovered with O adatoms, at varying degrees of CO and O coverage. Experimental evidence for the existence of islands was found. The results were described with an island model, that accounts for CO chemisorption from a precursor state, followed by either CO incorporation into CO islands or by a surface reaction with O adatoms from adjacent O islands. CO chemisorption was not activated and its rate coefficient was estimated as 40 s(-1), while the surface reaction was relatively fast. Indications for the size of the CO and O islands were found.