The temperature programmed desorption (TPD) is used to determine the experimental conditions needed to characterize the dispersion of precious metals by hydrogen chemisorption on three way exhaust gas catalysts. It is shown that hydrogen chemisorption on the samples is activated. An adsorption temperature of 370-473 K must be used in order to obtain a fast saturation of the sites. In these conditions, the support of the precious metals chemisorbs H2 as well. Only the TPD method permits to distinguish the two sites of adsorption. For the various fresh industrial catalysts studied the metal dispersion is around 100 % using a ratio H/metal=1. The change of the hydrogen chemisorption with the thermal ageing of the catalysts under inert gas is studied.