A new criterion to assess diesel fuels has been developed at the Institut Français du Pétrole, in collaboration with PEUGEOT and RENAULT. It is a comparative method which makes it possible to assign a Particulate Number (PN) to a diesel fuel, calibrated, as for the cetane number, with two reference fuels specially formulated. One has a low emission level, the other one a very high level. Values PN=0 and PN=100 are respectively attributed to each of them. The first is made up of tetradecane, and the second of a highly aromatic mixture containing 55 % tetradecane, 40 % 1-methyl-naphthalene and 5 % phenanthrene. The PN value of a diesel fuel derives from measurements of particulate emissions of an engine successively fuelled with the three products (CPP) and from a simple analysis in laboratory of the nitrogen content (N) of the tested fuel. The relation can be equated as follows: Validation work, undertaken on 5 cars and 2 sets of respectively 10 and 12 specially formulated diesel fuels, has shown that the PN index correlates well with the particulate emissions measured on vehicles during European standard test. These experiments show that this criterion could become a good predictive tool for particulate emissions.