Mathematical fitting of experimental emission data from five different fuels and three mixtures of biodiesel and reference diesel operating under diverse conditions allows to propose an easy equation to calculate both the insoluble and soluble fractions in Diesel Particulate Matter. The relative importance of each mechanism involved in the DPM formation process is also mathematically quantified. Two different mechanisms are principally found to take place during DPM formation. In a first stage (scrubbing effect), adsorbed sulphuric acid reacts with organic compounds in the exhaust to form heavy hydrocarbons which are likely to remain in the condensed phase. The remaining hydrocarbons undergo an absorption process and are retained on particles in the liquid phase. However, some light hydrocarbons may escape from the filter system and are not quantified as soluble organic fraction (SOF). The presence of two retention mechanisms makes it necessary to develop an accurate extraction technique to quantify the soluble organic fraction. (C) 2003 Elsevier Ltd. All rights reserved.