The noble metal catalysed oxidation of CO by O2 has been studied on polycrystalline foils, ribbons and wires of Pt, Pd and Rh as well as on a variety of supported Pt catalysts. At temperatures from 180 to 600-degrees-C, the partial pressure ratio was changed over the range 0.01 less-than-or-equal-to p(CO)/p(O)2 less-than-or-equal-to 50 at partial pressures varying over the range 10(-6) less-than-or-equal-to p(i) (mbar) less-than-or-equal-to 40. In the total pressure region below 1 mbar, the results agree very well with model experiments conducted under ultra-high vacuum. The rate values measured at total pressures exceeding 1 mbar correspond to the highest activities ever observed on noble metal catalysts. Even at low temperatures and high gas velocities up to 14 m s-1, such values were mass transfer limited since the reaction was not inhibited by CO. Neither the structure nor the support influence the reaction in the case of supported Pt catalysts. The well-defined geometry of the experimental set up allowed an estimation of the appropriate mass-transfer coefficients and the calculation of the intrinsic reaction rate. The results clearly indicate that in the region where the reaction is first order with respect to the limiting component the intrinsic first-order rate constant has the same value as under high vacuum conditions within the precision of the approach. Thus the apparent gap between the rates of CO oxidation observed in model experiments and under ambient pressure must be attributed to the influence of mass-transfer limitations.