A large number of candidate soot oxidation catalysts were screened on their catalytic activity with a model soot. It was found that the intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation rate. Two types of contact were studied; many catalysts increase the rate of soot oxidation considerably when the contact is intimate (’tight’), whereas under conditions of poor (’loose’) contact only some catalysts accelerate this oxidation reaction. It is tentatively suggested that (i) mobility of the catalyst is a major parameter determining the loose contact activity of catalysts and (ii) this mobility correlates with the melting point or the partial pressure of the catalyst.