The kinetics of oxidation of carbon monoxide on Pd-supported catalysts are determined at low temperature and under typical conditions of atmospheric pollution (a few ppm of CO in air). The nature (silica, alumina, silica-alumina) as well as the geometry (beads, fibers, powder) of the support were varied, the latter to change the mass-transfer resistance. The nature of the support has no influence on the kinetics. With powder or fiber as the supports, the catalyst operates without mass transfer limitation, and the kinetics is of negative order with respect to CO, of positive order with respect to oxygen. The energy of activation is about 85 kJ/mol, This corresponds to the regime of GO-inhibition, never reported at so high O-2 to CO ratio (75-10(4)). The surface is dominated by adsorbed carbon monoxide, and the reaction rate is limited by oxygen adsorption, With beads as the support, the orders in CO and in oxygen are both positive and the energy of activation is 42 kJ/mol, This kinetics law is attributed to mass transfer limitation.